Add unit tests for implicit fallthrough lint rule

Summary:
The perf fix actually catches some real problems.
I didn't find anything in libphutil, Arcanist and Phabricator though.

Also bump version.

Also allow configuring the hook.

Test Plan:
Added a test, saw it fail with the old code.
Repeat for hook.

Reviewers: epriestley

Reviewed By: epriestley

CC: aran, Korvin

Differential Revision: https://secure.phabricator.com/D5097