Fix plugin loading

Fix loading of plugins from the plugin directory.

In the process, change the default plugin directory name from
`.githooks` to `.git-hooks`. This will make migrating existing
repositories a bit easier since they'll be able to support plugins for
both the old and new Overcommit during the transition period.

Change-Id: I88c9573883e7b116c362dee03077cc715862019f
Allow updating commit message in commit-msg hooks

Add a helper `update_commit_message` to provide a standard interface for
updating commit messages via commit-msg hooks.

Change-Id: If7707b842db38460c1ae3d495543aca02a6326a4