Add short arguments for `limit-user` and `comments-per-pr`.
Add short arguments for `limit-user` and `comments-per-pr`.

Also:

* allow each user argument to be a comma separated list
* normalize user each argument by doing lowercase comparision
* return a user set rather than a list

balloob/farcy,appfolio/farcy,appfolio/farcy,balloob/farcy,balloob/farcy,appfolio/farcy