Parse command line input wrapped in a string

This is the simplest implementation of the functionality described in
the short description. This patch only deals with  the arguments seperated by
whitespaces, and process them as is, without any shell processing like
expanding wildcards or replacing variables.

include/cxxopts.hpp

@@ -1772,6 +1772,9 @@ namespace cxxopts
     ParseResult
     parse(int argc, const char* const* argv);
 
+    ParseResult
+    parse(std::string argv_str);
+
     OptionAdder
     add_options(std::string group = "");
 
@@ -2284,6 +2287,30 @@ Options::parse(int argc, const char* const* argv)
   return parser.parse(argc, argv);
 }
 
+inline
+ParseResult
+Options::parse(std::string argv_str)
+{
+  std::vector<std::string> tokens;
+  std::string token;
+  std::istringstream iss(argv_str);
+  while (std::getline(iss, token, ' '))
+  {
+    tokens.push_back(token);
+  }
+
+  const char** argv = new const char*[tokens.size()];
+  for (int i = 0; i < tokens.size(); ++i) {
+    argv[i] = tokens[i].c_str();
+  }
+
+  OptionParser parser(*m_options, m_positional, m_allow_unrecognised);
+
+  auto parse_result = parser.parse(tokens.size(), argv);
+  delete argv;
+  return parse_result;
+}
+
 inline ParseResult
 OptionParser::parse(int argc, const char* const* argv)
 {