Add --force flag to always overwrite hooks

If a user attempted to install Overcommit hooks into a git repository
that already had non-overcommit hooks, it would report an error. Allow
the user to specify a `--force` flag which disables this error and
ensures the hooks are installed, overwriting if necessary.

Closes #7

Change-Id: I9390e5e122572444202f0b1ef8ae172a15c5b4dc
Reviewed-on: http://gerrit.causes.com/36004
Tested-by: jenkins <jenkins@causes.com>
Reviewed-by: Shane da Silva <shane@causes.com>