make sure this works with existing hooks

[koddsson]

Right now we just exit if there is a commit-msg hook already which is good, this means that we don't overwrite existing hooks.

It would be really cool if we could temporarily install the hook along side the other hooks that are already installed.

Also add a --no-hooks flag that won't install any hooks and will assume you already have them so that people can at least just copy paste the code into their existing hook.