Exclude code based on file name and not on class/package name

Closes #120.

Resolves #88.

Reverts and fixes #71.

@puffnfresh, please review. :smiley_cat:

Some things to consider:

1. It's using the same `wartremoverExcluded` sbt option, but I doubt people were using the previous non-working exclusions. Or in compiler plugin output?
1. Maybe we should warn in sbt's output if a `File` provided in `wartremoverExcluded` is non-existent?