Add hook for runtime config settings.

Summary:
This adds a hook to allow external parties to provide config settings at runtime.
The hook is technically for when a RepositoryAPI is created, but that moment can
be used to set new config settings using the new setRuntimeConfig() api.

For example you could have a external hook that looks for keys like 'git:foo.bar'
or 'hg:foo.bar' and writes the value of 'foo.bar' based on whether the repo is a
git or a hg repo.

Test Plan:
Created a hook that looks for hg/git prefix versions of config keys.
Set hg:arc.feature.start.default to be "master" and set arc.feature.start.default
to be "trunk".
Ran arc feature in the hg repo.  It made a bookmark on master.
Ran arc feature in the git repo.  It made a branch on trunk.

Did it again, but with git:arc.feature... set instead.

Reviewers: epriestley

Reviewed By: epriestley

CC: nh, wez, aran, Korvin

Differential Revision: https://secure.phabricator.com/D5357