Support for Guard 2.x plugin syntax

Guard 2.x introduced small changes to the API that affect how Guard plugins are created. 

This PR implements the required changes that are detailed in the [Upgrading to Guard 2.0 wiki](https://github.com/guard/guard/wiki/Upgrading-to-Guard-2.0#guardguard-deprecated-in-favor-of-guardplugin).
- [x] Inheriting from `::Guard::Guard` has been deprecated in favor of `::Guard::Plugin`
- [x] The `#initialize` signature has changed

I've also bumped the version number to `2.0.0` to reflect the compatibility with Guard version.

P.S. Hope you are doing well!