Allow specifying runtime configuration with --set-config key=value

Summary:
This is useful for wrapper scripts that want to customize arcanist's behavior without affecting the global configuration.
This can be implemented with arcanist_configuration entry in .arcconfig, however it is currently limited to
per-project settings, and this feature makes writing wrapper scripts a little easier.

Test Plan: arc diff --set-config editor=vim (yeah yeah, crappy test case)

Reviewers: epriestley, #blessed_reviewers

Reviewed By: epriestley, #blessed_reviewers

Subscribers: epriestley, Korvin

Differential Revision: https://secure.phabricator.com/D9442