Support WEBM format for lightweight video tutorials

[bloxster]

I need to insert short video tutorials, but I see that the only format supported is GIF, which is not ideal for compression. Supporting the WEBM format instead would allow me to save a significant amount of space. For example, a 15-second screencast weighs 15MB in GIF format but only 1MB in WEBM format.

I am now using the external free WEBM hosting service webm.red which seems to work well with the embedding in Gitbook, but a full WEBM integration would be cool.