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?
iiam
Reviewers: btrahan, vrana, jungejason
Reviewed By: btrahan
CC: aran
Differential Revision: https://secure.phabricator.com/D2400
