Add PreCommit::PreventDebugger

Closes #7
I learned a few things from these lines alone:

1. `Regexp::union`.
2. `binding.remote_pry`.
3. `#save_and_open_page`.

I don't agree with the decition of not allowing `logger.debug`. It may be useful to leave some debug level logs in production code. Then, when some nasty bug arrises, the log level in production processes can be changed for a while to track it down.