Add optional limiting to AR backend

Background here is that our prod db has 45k records in the delayed_jobs table so we didn't want to load them all onto the page, added an optional config that will limit if set, if not do nothing.

Hoping you'll find this a useful addition so we can keep using your awesome gem directly instead of needing to use our fork! Let me know if there's anything else needed to get it up to snuff. I didn't add any tests but I did verify that the tests still pass (at least the AR ones, don't have mongodb installed to check but the changes there are very noninvasive) but if needed I could probably figure out a way to cover the feature with a new test, it just might not be pretty since it's a configuration setting.

Either way, thanks again for the great gem!


Sincerely,
John Wilkinson
Software Engineer at Lumosity