Add ability to index an externally maintained repository

Related to issue #207. Right now you can specify a "git" repository and it'll clone/fetch and update the index if the HEAD changes.

I already have a system to deploy and keep Git repositories updated, and would prefer not to deal with another one, so if the clone/fetch calls were no-opped via some config option then hound would poll whether "HEAD" changed and trigger re-indexes as appropriate.

Right now the only way to do this is to make hound's $PATH have a dummy "git" that no-ops clone/fetch but passes through other commands.