Support editing existing comments instead of overwriting

I think it'd be nice (maybe it happens on `danger.rb`?) for problems that are resolved to be struck out? It makes it hard to track progress and reasons for certain commits.

In this, I'd also like to see some logic to check if the comment if actually a danger comment before overwriting. We use the same bot on a bunch of repos, and it comments about various things in various places. It took me a while to realise that it was still working, but danger was overwriting the comment instead of creating a new one.