Add "arc get-config" and "arc set-config" for managing ~/.arcrc values

Summary:
The major thing I want to do here is allow you to set a default Phabricator URI, so we can make "arc paste", and "arc upload", "arc download" work anywhere.

We can also relax the .arcconfig requirements (request from @csilvers).

Test Plan:
Get/set some values?

Reviewers: btrahan, vrana, jungejason

Reviewed By: btrahan

CC: aran

Differential Revision: https://secure.phabricator.com/D2400