Add a parser extension to provide custom methods for Parser

I defined a `noSuchMethod` in `Parser`, and it will check a `parserExtension` object for missing methods.

The `parserExtension` object can be set in client code, and user can define a class with some extension methods.

There is a test file in the commit, and you can get my idea.

I haven't written the doc, and I hope to get your advice on the idea. Thank you!