Merge Reporter into HookRunner

The `HookRunner` and the `Reporter` class were initially separated to
try and decouple the logic of displaying results from the logic of how
to run the actual hooks.

Unfortunately, these two concepts are tightly-related. For now, merge
them together.

Change-Id: I6f284c0fdf4a619c91ede53e04c06f3445d1d4c4
Reviewed-on: http://gerrit.causes.com/35612
Reviewed-by: Shane da Silva <shane@causes.com>
Tested-by: Shane da Silva <shane@causes.com>
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
Reviewed-on: http://gerrit.causes.com/35613
Reviewed-by: Shane da Silva <shane@causes.com>
Tested-by: Shane da Silva <shane@causes.com>