File Specificity Overhaul

We can maintain it indefinitely if that's a need, but eventually it should become opt-in, I think. The process would be:

* Add a warning and foodcritic rule when files are found in `default`. Ideally this is done in a X.0.0 release, but if that's impractical we can do it in a X.Y.0 release. You can set a config option to confirm the default, which turns off the warning.
* Change the default in a X.0.0 release.