Allow `willLintPaths` and `didLintPaths` to be overridden
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

Khan/arcanist,phacility/arcanist,aik099/arcanist,aik099/arcanist,codelabs-ch/arcanist,phacility/arcanist,aik099/arcanist,Khan/arcanist,aik099/arcanist,Khan/arcanist,phacility/arcanist,codelabs-ch/arcanist,codelabs-ch/arcanist,codelabs-ch/arcanist,Khan/arcanist,phacility/arcanist,uber/arcanist,uber/arcanist,codelabs-ch/arcanist,uber/arcanist,phacility/arcanist,uber/arcanist