Allow linters that extend ArcanistExternalLinter to customize the file parameter
Summary: Not all linters run on a command file fashion. In particular, the maven checkstyle plugin runs like command --flag=file.
Test Plan: Run a linter that extends ArcanistExternalLinter.
Reviewers: Blessed Reviewers, epriestley
Reviewed By: Blessed Reviewers, epriestley
Subscribers: epriestley, Korvin
Differential Revision: https://secure.phabricator.com/D9026