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