Added markdown extension for youtube

the syntax to embed a youtube video is

![WIDTHxHEIGHT](youtube:YOUTUBEID)

For example: ![450x230](youtube:qCW-SVPCw4Y)
will generate following HTML:
<p><iframe width="450" height="230"
src="//www.youtube.com/embed/qCW-SVPCw4Y" frameborder="0" allowfullscreen></iframe></p>

width and height are optional, you can leave the brackets empty, then
the default size would be 560x315

To render markdown just call Markdown.render(your_markdown)