Fix positional arguments overload

Fixes #83.

Adds an overload for positional arguments taking a
`std::initializer_list`. When using an `initializer_list` with one
argument, the function call is ambiguous matching both `std::string`
and `std::vector<std::string>`.

test/options.cpp

@@ -145,7 +145,7 @@ TEST_CASE("All positional", "[positional]")
   auto argc = av.argc();
   auto argv = av.argv();
 
-  options.parse_positional("positional");
+  options.parse_positional({"positional"});
 
   auto result = options.parse(argc, argv);