Add commit message hook, starting with line length

They're just warnings for now (big yellow text), but soon they may be
errors.

Change-Id: Ib3aa5079d9db40fdb0ab32ea335931a317ac7612
Reviewed-on: https://gerrit.causes.com/22209
Tested-by: Aiden Scandella <aiden@causes.com>
Reviewed-by: Joe Lencioni <joe.lencioni@causes.com>