Fix constness of Options::parse

include/cxxopts.hpp

@@ -1414,6 +1414,9 @@ namespace cxxopts
     ParseResult
     parse(int argc, const char** argv);
 
+    ParseResult
+    parse(int argc, char** argv);
+
     OptionAdder
     add_options(std::string group = "");
 
@@ -1872,6 +1875,15 @@ Options::parse(int argc, const char** argv)
   return parser.parse(argc, argv);
 }
 
+inline
+ParseResult
+Options::parse(int argc, char** argv)
+{
+  OptionParser parser(*m_options, m_positional, m_allow_unrecognised);
+
+  return parser.parse(argc, const_cast<const char**>(argv));
+}
+
 inline ParseResult
 OptionParser::parse(int argc, const char** argv)
 {