Multi package linting

This should fix #68. I still want to figure out if it is a problem that foo and foo_test are linted together but I would also like to hear your feedback if this is an OK direction.
Do we need to change lintFiles? I would assume if you want to lint multiple packages then you list multiple packages (maybe via `./...`) rather than listing disparate files. The usage instructions explicitly say that if you are listing files they should be in a single package.