Add "disable autoplay" option for Livestream shortcode

[goldenapples]

Currently, the default behavior for Livestream embeds is to autoplay video. This may not be desirable for sites, and it would be good to add a "disable autoplay" option to the [livestream] shortcode.

This is reflected on the shortcode's output by the autoPlay query string parameter, which defaults to true if not set.

Normal, auto-play enabled URL:

https://livestream.com/accounts/12767816/events/4512162/videos/105580270/player/

Modified auto-play disabled URL:

https://livestream.com/accounts/12767816/events/4512162/videos/105580270/player/?autoPlay=false