Add PreCommit::PreventDebugger

Closes #7
@rranelli I understand your point. But don't you think this verification is too restrictive and tries to outsmart the developer? It seems strange that reasonable code should require a `--no-verify` flag. I would rather think of this flag as a way of saying "I know I'm being nasty and OK with that".

I believe leaving `binding.pry` in the code is not under the same hat as leaving `logger.debug`.