Improve handling of options for arc workflows

Summary: Allow --severity=warning to mean the same as --severity warning. Longer term, we should convert this code to use PhutilArgumentParser, although it doesn't seem that PhutilArgumentParser support British spelling ;)

Test Plan: Ran arc lint --severity=warning. Also var_dumped $args to make sure it looked reasonable.

Reviewers: Blessed Reviewers, epriestley

Reviewed By: Blessed Reviewers, epriestley

Subscribers: Korvin, epriestley

Differential Revision: https://secure.phabricator.com/D11464