Added gfycat support. Anonymous uploads only, supports gif and ffmpeg video

Gfycat is an image host for gifs and short-soundless-videos, it provides much better hosting and playback than gifs on most browsers by using html5 where available. Generally, uploading gifs to imgur or videos to a file host will result in worse playback than uploading them to gfycat, so I've added support for it as an uploader.

The one caveat is that the ShareX uploader is a file uploader and not an image uploader. This is because it supports ffmpeg, which is considered a file not an image. This could possible be changed in the future by asking each target uploader if they support a given type, before trying to guess it ourselves.