Add setting to turn "autoplay" on/off for youtube/vimeo links.

* Allow users to turn "autoplay" for youtube/vimeo on/off
* Fix multiple videos playing at once when opening self post containing multiple videos (only first video in post should autoplay (assuming auto-play is enabled).

Fixes https://github.com/honestbleeps/Reddit-Enhancement-Suite/issues/879

Sort of fixes https://github.com/honestbleeps/Reddit-Enhancement-Suite/issues/875 (but may not be best solution long term)