Add a writeLog() method to PhutilConsole for writing debug/log information

Summary: Add a new console channel which we dump to stderr if --trace is set. In the future, we could refine this behavior.

Test Plan: Implemented the arcanist top level in terms of this; see next diff.

Reviewers: nh, vrana

Reviewed By: vrana

CC: aran

Differential Revision: https://secure.phabricator.com/D3422