Allow willLintPaths and didLintPaths to be overridden

Summary: I'm not sure if the upstream will be interested in this change, but we are writing a linter which works by running an external command on the entire repository. This can't be done with ArcanistExternalLinter at the moment, which meant that we ended up copy-pasting most of ArcanistFutureLinter. This would be a lot easier if we could override willLintPaths and didLintPaths, but these methods are currently marked as final. An alternative solution would be some sort of ArcanistLinter::transformPath method.

Test Plan: N/A

Reviewers: epriestley, Blessed Reviewers

Reviewed By: epriestley, Blessed Reviewers

Subscribers: faulconbridge, Korvin

Differential Revision: https://secure.phabricator.com/D19630