Allow spaces in option specification. (#58)

Fixes #57.

Allows spaces after the comma between the short and long option
specification.

include/cxxopts.hpp

@@ -884,7 +884,7 @@ namespace cxxopts
       ("--([[:alnum:]][-_[:alnum:]]+)(=(.*))?|-([[:alnum:]]+)");
 
     std::basic_regex<char> option_specifier
-      ("(([[:alnum:]]),)?([[:alnum:]][-_[:alnum:]]*)?");
+      ("(([[:alnum:]]),)?[ ]*([[:alnum:]][-_[:alnum:]]*)?");
 
     String
     format_option

src/example.cpp

@@ -38,7 +38,7 @@ int main(int argc, char* argv[])
     options.add_options()
       ("a,apple", "an apple", cxxopts::value<bool>(apple))
       ("b,bob", "Bob")
-      ("f,file", "File", cxxopts::value<std::vector<std::string>>(), "FILE")
+      ("f, file", "File", cxxopts::value<std::vector<std::string>>(), "FILE")
       ("i,input", "Input", cxxopts::value<std::string>())
       ("o,output", "Output file", cxxopts::value<std::string>()
           ->default_value("a.out")->implicit_value("b.def"), "BIN")

test/options.cpp

@@ -52,6 +52,7 @@ TEST_CASE("Basic options", "[options]")
     ("value", "an option with a value", cxxopts::value<std::string>())
     ("a,av", "a short option with a value", cxxopts::value<std::string>())
     ("6,six", "a short number option")
+    ("p, space", "an option with space between short and long")
     ;
 
   Argv argv({
@@ -62,7 +63,9 @@ TEST_CASE("Basic options", "[options]")
     "value",
     "-a",
     "b",
-    "-6"
+    "-6",
+    "-p",
+    "--space",
   });
 
   char** actual_argv = argv.argv();
@@ -77,6 +80,8 @@ TEST_CASE("Basic options", "[options]")
   CHECK(options["value"].as<std::string>() == "value");
   CHECK(options["a"].as<std::string>() == "b");
   CHECK(options.count("6") == 1);
+  CHECK(options.count("p") == 2);
+  CHECK(options.count("space") == 2);
 }
 
 TEST_CASE("Short options", "[options]")