Improve rubocop todo

I've been thinking that we should be able to do better than just generating `Enabled: false` in `rubocop-todo.yml`. Here's an implementation that sets `Max` (for all cops that have a `Max`) and `EnforcedStyle` (for some that have it).

So I haven't gone all the way and implemented the functionality for all configurable cops. I wanted to see what people think of it first. It does require a little bit of code added to the cops.