Option to include additional files

[john-kurkowski]

This addon has the excludeFiles option, but no includeFiles. Could you add it? So projects can opt into linting important project files beyond ember-cli's magic app/ and tests/ directories.

For example, samselikoff/ember-cli-mirage#49 moves the mirage/ folder into the root of the project. I would like to tell broccoli-jscs to lint mirage/ in addition to ember-cli's magic directories.

(Related to but maybe the opposite of #43?)