Make the behavior of "--load-phutil-library" more explicit

Summary:
This flag disables normal libraries to make it easier to test library changes. Print that out explicitly.

Also update `.arcconfig` to use the modern configuration format.

Test Plan: Ran `arc list --trace`, `arc list --load-phutil-library=src --trace`.

Reviewers: zeeg, btrahan, vrana

Reviewed By: btrahan

CC: aran

Differential Revision: https://secure.phabricator.com/D5768
Merge branch 'master' of http://github.com/facebook/arcanist