Use `file` to determine which files are Python files

[OddBloke]

This also refactors the pdb checker to use the same grep command as the grep checker, and files_for_commit to return a list of filenames (rather than a bash object).

[alexcouper]

For future reference, I've begun adding test files for each of the checkers.

https://github.com/alexcouper/captainhook/blob/master/test/test_checkers/test_pdb.py