Add a very basic Scala (SBT) linter.

Summary:
SBT is the most common Scala buildsystem. This adds an extremely basic and
slightly horrible linter to check SBT's output for warnings and errors.

Test Plan:
Tested this with a Scala project I've been working on for some time.
It seemed relatively sane.

Reviewers: epriestley

Reviewed By: epriestley

CC: aran, Korvin

Differential Revision: https://secure.phabricator.com/D4064