defaulthandler in options of v2.x

[gonenduk]

Add an option for a default (fallback) handler to be used if no other handler was found or given:

• Default handler is optional.
• Handler must be a function.
• Default handler is a fallback and is called only if handlers (object or files) doesn't have the right handler.

NOTE 1: in version 1.x there was an option with the same name which was used not as a fallback. It was always used for all paths and methods - making it quite useless.
The new option use case might be a 501 "not implemented" error (instead of 404 we have today) or any other default behaviour.

NOTE 2: I only made the code changes. If you approve the change, I will update the readme doc as well with explanations and samples.

[gonenduk]

@subeeshcbabu