Add --run flag

Add `--run` flag for running overcommit against all git tracked files.
This allows for overcommit to be integrated in a CI setup.

Change-Id: Ic5020aef1b1dcf6ebc07dea103e58074605bea4f
Reviewed-on: http://gerrit.causes.com/44653
Tested-by: jenkins <jenkins@brigade.com>
Reviewed-by: Shane da Silva <shane.dasilva@brigade.com>
Update README with --run flag

Change-Id: Ie0c30875e973e3b00ebe04a2b7c58146fb631351
Reviewed-on: http://gerrit.causes.com/44654
Tested-by: jenkins <jenkins@brigade.com>
Reviewed-by: Shane da Silva <shane.dasilva@brigade.com>