Set limits on the number of comments posted for each issue.

This is my attempt at implementing #33. I'm referring to it as a "comment limit" instead of just "limit" since the issue will still be updated with the last occurrence time/occurrence count, it's just the comment that won't be added.

Building on this idea, it would be cool to be able to throttle an issue (e.g. only report/comment on an occurrence once per minute or hour, etc.)