Add support for Video

[cloverich]

In my current branch I've added support for drag and drop images: Drag an image into a document, it uploads to a folder and embeds into the document. Supporting video files in the same format almost works. After some tweaking, they both upload and embed correctly. However, when saving the document they do not save because the parser does not know how to deal with the type.

• Figure out how to add video handling to the parser (views/edit/util SlateTransformer.nodify)