Filter by mime_type on Media Endpoint

[dshanske]

One of the attempt to decide what returns q=source on the Media Endpoint would look like, @aaronpk #14 (comment) added url, published, and mime_type. I also have mime_type in my return now.

So, this proposal would allow q=source to accept the optional parameter of mime_type, to only return media of a certain mime-type. Mime types consists of a type and a subtype. The parameter would accept both, or just the type...so mime_type=image would return every subtype of image and so on.