Allow a "types" parameter on the mentions endpoint

[quentez]

The mentions endpoint already return objects with a type property. Therefore it wouldn't be very expensive or anything to allow filtering based on type.

There's plenty of use-cases for this, here's one of the most obvious: I want to differentiate between the number of mentions that are reposts and those that are favorites. How can I display a repost count for a post without this? How can I display the 3 first reposts without this? etc.