Add --exclude-dir to complement dir

[Daenyth]

So that I can astpath "foo" --dir=pkg/ --exclude-dir=pkg/junk

[hchasestevens]

@Daenyth While this is still yet-to-be implemented, please see https://github.com/hchasestevens/bellybutton , which does include this functionality as part of a larger overall linting tool.