Add sniff for file name conventions

[jaymcp]

This sniff is an extension of WordPress' filename sniff, to include support for abstract classes.
If one had a file such as this:

<?php

abstract class Foo_Bar {}

WordPress' sniff would want the file name to be class-foo-bar.php.
This sniff would want the file name to be abstract-class-foo-bar.php.

There are also some cleanup operations included in this PR.

[jaymcp]

thanks for approving, @jrmd. i'm going to merge this. if there's any feedback forthcoming from the other reviewers, i can make follow up PRs