Add a category option

I have a case where there are several Prop limits that fall into the same "category" of limitation, and I want to be able to determine that category quickly in the `rescue` clause for the `Prop::RateLimited` exception.

Right now, the only option is to check the handle against a list of members of that category. This list has the potential to get out of sync with the actual list of `Prop.configure` calls, and worse, in this case, the list wouldn't even be in the same gem as the `Prop.configure` calls.

If I could pass the same category for all of these limits when calling `Prop.configure`, then in the `rescue` clause, I could just check the category and not have to have a separate list.

The category param will be optional so this shouldn't disrupt any existing usages of prop.

@morten @jish @craiglittle @grosser @steved555