Lint commented out code.

The goal is to prevent people from committing/pushing code that has been commented out, which is basically equivalent to dead code. Instead, they should remove the code and use their version control system if they want to recover it in the future.

This should for obvious reasons not trigger for doc comments, and only for comments that contain only code.

I have no good idea how to determine whether a comment is a "code that has been commented out" comment except for trying to parse it.