Adding arc lint flag that will show lint only on changed lines - even when paths are specified

Summary:
The existing lint configurations do not allow for linting only the lines that have
changed when paths are added.  --lintall has a default behavior of true when paths are specified,
and false when paths are not specified.  Because of this (and because it does not take a boolean
param) it is not possible to lint a path for only the errors on changed lines - only-new is not
working presently.

Test Plan: play around with the linter

Reviewers: lifeihuang

Reviewed By: lifeihuang

CC: epriestley, aran

Differential Revision: https://secure.phabricator.com/D7055