RFC: API to cover both adding and removing files from coverage

[kellyselden]

Now that we are requiring all files, issues like #65, #66, and #75 are popping up, and the problem may not be immediately clear. We also have files we want to stop covering in #71. I propose we unify any API to resolve these issues, or at least make them compatible. Maybe both a shouldInclude and a shouldExclude hook. That's about as far as I got in the thought process.

[RobbieTheWagner]

@kellyselden there is an exclude array, but I do not think it reliably works. We could have a sane exclude default, and then the user could override it to be blank if they wanted to include everything or add to it to exclude more, etc. Thoughts?