Adds support for ?list= in YouTube URLs

Adds support for YouTube URLs of the form **//www.youtube.com/embed/foo?list=bar**, where **bar** is a playlist and **foo** is the first video in that playlist to be played. Usage is thus:

    video::foo[youtube,list=bar]

For example, https://www.youtube.com/watch?v=79gAss0K1TI&list=PLhQjrBD2T382Lqs7bsMl6WRDA9anaEzBe could thus be embedded with:

    video::79gAss0K1TI[youtube,height=315,list=PLhQjrBD2T382Lqs7bsMl6WRDA9anaEzBe,width=560]

which would yield:

    <iframe width="560" height="315" src="//www.youtube.com/embed/79gAss0K1TI?rel=0&list=PLhQjrBD2T382Lqs7bsMl6WRDA9anaEzBe" frameborder="0" allowfullscreen></iframe>