Add poster attribute of the video element to the staticMap

[leechy]

Good morning.
Right now poster attribute if the video element is not rewrited (prefixed properly), which leads to 404-errors.
Hard to notice, because the video itself is loaded fast enough in most cases on 4game project.

https://github.com/sergeche/html-transform/blob/master/lib/rewrite-url.js#L79