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