Allow `ArcanistExternalLinter` flags to be specified as an array.

Summary: Personally, I prefer to specify command lines flags as an array rather than a string.

Test Plan: `arc unit`

Reviewers: epriestley, #blessed_reviewers

Reviewed By: epriestley, #blessed_reviewers

Subscribers: aran, epriestley, Korvin, chad

Differential Revision: https://secure.phabricator.com/D8387