[Feature] embed video

[Thiago-Simoes]

Currently embed video is not in markdown standard, but many editors have support for videos, hosted on youtube/vimeo.

Proposal:

If user insert link to youtube/vimeo video, for example:

[!embed](http://www.youtube.com/watch?v=someid)
it should be rendered as

<iframe width="480" height="270" src="youtube|vimeo-hosted-embed-video-url" frameborder="0" allowfullscreen></iframe>  

User should be able to configure the maximum size of the embedded content with the max_height and / or max_width parameters. Youtube widescreen mode can be controlled via widescreen param.

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Replicating the shsmad issue