Refactor ModifiedFile to CommitFile

Generalize the name to handle files that may not have been modified, like
`.hound.yml`.
The idea I'm going for is that `CommitFile` is a convenience object that gets created with a hash of attributes.
This could be an option, but that feels weird that a `CommitFile` is initialized with some `file` object plus its contents.
```ruby
CommitFile.new(file, file_contents(file.filename)
```

Thoughts?