Add pass_filenames support - #265

@asottile could you take a look please?

as discussed offline -- I don't really want to encourage pass_filenames: false -- it's usually an indication that the tool is not a good fit for pre-commit (since one of the main points of pre-commit is to treat units of work as filename-based and skip when necessary, handle merge conflicts properly, partial staged contents, etc.). pass_filenames: false also often needs always_run: true which is also a smell.

That's fair. The only other argument I can think of is that the mirror should support all arguments as the hook definition "just because". Also I can accomplish what I want with a local hook (altho I have to manage versioning on my own)