Allow checks to provide a extension matcher

When a check is for a rst file vs a plaintext
file and is not interoperable this change allows
that check to declare that it only knows how to
scan certain files.

It also fixes the max line checker to use a similar
routine to differentiate between rst files and non
rst files and uses a different routine based on this
decision.

Change-Id: I1e3e5f9f1631ed5babe2df17c987e8e872938a38
Merge "Allow checks to provide a extension matcher"