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