Don't show default when boolean false

Fixes #102. Don't show the default value when it is a boolean and the
value is false. Note that this is a bit of a hack and the
implementation should probably be reevaluated in the future.

include/cxxopts.hpp

@@ -1372,7 +1372,7 @@ namespace cxxopts
     {
       auto desc = o.desc;
 
-      if (o.has_default)
+      if (o.has_default && (!o.is_boolean || o.default_value != "false"))
       {
         desc += toLocalString(" (default: " + o.default_value + ")");
       }

src/example.cpp

@@ -40,6 +40,7 @@ int main(int argc, char* argv[])
     options.add_options()
       ("a,apple", "an apple", cxxopts::value<bool>(apple))
       ("b,bob", "Bob")
+      ("t,true", "True", cxxopts::value<bool>()->default_value("true"))
       ("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>()