Add arcconfig setting for default arc feature start

Summary:
Adds arc.feature.start.default arcconfig setting to specify
a default value for 'start' in 'arc feature name start'. This lets
users always branch from origin/master (or whatever the main branch is).

Also cleaned up the 'feature' help text a little. The stuff about sorting
and closed/abandoned revisions is explained via the options list already.

Test Plan:
Ran arc feature with/without a start and with/without the config
setting set.

Reviewers: epriestley, vrana

Reviewed By: epriestley

CC: bos, sid0, dschleimer, aran, Korvin

Differential Revision: https://secure.phabricator.com/D5184