Adds support for passing the modified paths as arguments to the rake task

Hi,

I suppose this is the opposite of [using guard programmatically in a Rake task](https://github.com/guard/guard/wiki/Guard-Cookbook).

I set the new options' defaults to not impact current users of guard-rake. Their interaction is a bit confusing though, let me know if I should be more thorough in the README.

That said, the Guard-Cookbook way may be the preferred approach.

Cheers