Improve formatting of help descriptions (#213)

* new function: cxxopts::Option::set_width(size_t width)
  Set the size of a helpline.
* new function: cxxopts::Option::set_tab_expansion()
  Expand the tabs in descriptions.
  The tabsize 8 chars, base is start of description.
  The descriptions are not disturbed by adding additional options.
* Allow newlines \n nad tabs \t in descriptions.

Other changes (last commit/new commit):
* 1453/1471: size_t for OPTION_LONGEST and OPTION_DESC_GAP.
  This prevents the static cast in 2086/2140.
* 2088/2142: in case of small width the value of
  "width - longest - OPTION_DEC_GAP" becomes negative.
  Because size_t is unsigned the result is a big number, and
  the width of the column of the descriptions is not shortened.
* new 2143: When the given width is too small, it is set to
  longest + OPTION_DESC_GAP + 10
* new 1570: A long description is broken into multiple lines, and
  the iterator lastSpace remembers the begin of the last word.
  But when the iterator current reaches the end of line, the whole
  string from iterator is printed, which in soome cases is too
  long. Thats why one blank is added to the description to trigger
  the handling of lastSpace.
  Accordingly in 1574/1627 the line is shortened by one char.

include/cxxopts.hpp

@@ -1297,6 +1297,8 @@ namespace cxxopts
     , m_positional_help("positional parameters")
     , m_show_positional(false)
     , m_allow_unrecognised(false)
+    , m_width(76)
+    , m_tab_expansion(false)
     , m_options(std::make_shared<OptionMap>())
     , m_next_positional(m_positional.end())
     {
@@ -1330,6 +1332,20 @@ namespace cxxopts
       return *this;
     }
 
+    Options&
+    set_width(size_t width)
+    {
+      m_width = width;
+      return *this;
+    }
+
+    Options&
+    set_tab_expansion(bool expansion=true)
+    {
+      m_tab_expansion = expansion;
+      return *this;
+    }
+
     ParseResult
     parse(int& argc, char**& argv);
 
@@ -1414,6 +1430,8 @@ namespace cxxopts
     std::string m_positional_help;
     bool m_show_positional;
     bool m_allow_unrecognised;
+    size_t m_width;
+    bool m_tab_expansion;
 
     std::shared_ptr<OptionMap> m_options;
     std::vector<std::string> m_positional;
@@ -1450,8 +1468,8 @@ namespace cxxopts
 
   namespace
   {
-    constexpr int OPTION_LONGEST = 30;
+    constexpr size_t OPTION_LONGEST = 30;
-    constexpr int OPTION_DESC_GAP = 2;
+    constexpr size_t OPTION_DESC_GAP = 2;
 
     std::basic_regex<char> option_matcher
       ("--([[:alnum:]][-_[:alnum:]]+)(=(.*))?|-([[:alnum:]]+)");
@@ -1506,7 +1524,8 @@ namespace cxxopts
     (
       const HelpOptionDetails& o,
       size_t start,
-      size_t width
+      size_t width,
+      bool m_tab_expansion
     )
     {
       auto desc = o.desc;
@@ -1525,53 +1544,85 @@ namespace cxxopts
 
       String result;
 
+      if (m_tab_expansion)
+      {
+        String desc2;
+        auto size = size_t{ 0 };
+        for (auto c = std::begin(desc); c != std::end(desc); ++c)
+        {
+          if (*c == '\n')
+          {
+            desc2 += *c;
+            size = 0;
+          }
+          else if (*c == '\t')
+          {
+            auto skip = 8 - size % 8;
+            desc2.append(skip, ' ');
+            size += skip;
+          }
+          else
+          {
+            desc2 += *c;
+            ++size;
+          }
+        }
+        desc = desc2;
+      }
+
+      desc += " ";
+
       auto current = std::begin(desc);
       auto startLine = current;
       auto lastSpace = current;
 
-      auto size = size_t{};
-
       while (current != std::end(desc))
       {
-        if (*current == ' ')
+        size_t addNewLine = 0;
+        while (*current == '\n')
         {
+          *current = ' ';
+          ++current;
+          ++addNewLine;
+        }
+        if (addNewLine == 0 && current - startLine >= width)
+        {
+          if (lastSpace != startLine)
+          {
+            current = lastSpace;
+          }
+          if (current + 1 != std::end(desc))
+          {
+            ++addNewLine;
+          }
+        }
+        if (addNewLine > 0)
+        {
+          stringAppend(result, startLine, current);
+          startLine = current;
           lastSpace = current;
+
+          while (addNewLine-- > 0)
+          {
+            stringAppend(result, "\n");
+          }
+
+          if (current + 1 != std::end(desc))
+          {
+            stringAppend(result, start, ' ');
+          }
         }
 
-        if (*current == '\n')
+        if (std::isblank(*current))
         {
-          startLine = current + 1;
+          lastSpace = current + 1;
-          lastSpace = startLine;
-        }
-        else if (size > width)
-        {
-          if (lastSpace == startLine)
-          {
-            stringAppend(result, startLine, current + 1);
-            stringAppend(result, "\n");
-            stringAppend(result, start, ' ');
-            startLine = current + 1;
-            lastSpace = startLine;
-          }
-          else
-          {
-            stringAppend(result, startLine, lastSpace);
-            stringAppend(result, "\n");
-            stringAppend(result, start, ' ');
-            startLine = lastSpace + 1;
-          }
-          size = 0;
-        }
-        else
-        {
-          ++size;
         }
 
         ++current;
       }
 
       //append whatever is left
-      stringAppend(result, startLine, current);
+      stringAppend(result, startLine, std::end(desc) - 1);
 
       return result;
     }
@@ -2082,11 +2133,14 @@ Options::help_one_group(const std::string& g) const
     longest = (std::max)(longest, stringLength(s));
     format.push_back(std::make_pair(s, String()));
   }
+  longest = (std::min)(longest, OPTION_LONGEST);
 
-  longest = (std::min)(longest, static_cast<size_t>(OPTION_LONGEST));
+  //widest allowed description -- min 10 chars for helptext/line
-
+  size_t allowed = 10;
-  //widest allowed description
+  if (m_width > allowed + longest + OPTION_DESC_GAP)
-  auto allowed = size_t{76} - longest - OPTION_DESC_GAP;
+  {
+    allowed = m_width - longest - OPTION_DESC_GAP;
+  }
 
   auto fiter = format.begin();
   for (const auto& o : group->second.options)
@@ -2097,7 +2151,7 @@ Options::help_one_group(const std::string& g) const
       continue;
     }
 
-    auto d = format_description(o, longest + OPTION_DESC_GAP, allowed);
+    auto d = format_description(o, longest + OPTION_DESC_GAP, allowed, m_tab_expansion);
 
     result += fiter->first;
     if (stringLength(fiter->first) > longest)