Add option to check directories

[grahl]

In projects with a large amount of yaml files it can take an inordinate amount of time to lint them since the overhead of starting yaml-cli incurs a lot of overhead.

With ~2'500 files in a BLT project we often see about 2min spent on parsing, with this change it's closer to 2s.

[coveralls]

Coverage decreased (-4.2%) to 88.889% when pulling 741db9a on grahl:master into 51d8ecf on grasmash:master.