Add output-json option to arc feature

Summary:
The current format of arc feature is not very friendly to parsing.  This adds a json output
format.

Test Plan: Run the command and ensure output is valid

Reviewers: lifeihuang, #blessed_reviewers, epriestley

Reviewed By: epriestley

CC: epriestley, aran, Korvin

Differential Revision: https://secure.phabricator.com/D7647