Allow media/youtube `enable` functions to also accept an array of `id`s

[greg-el]

Currently id is required to be a string, meaning the function (either enableYouTubeTracking or enableMediaTracking has to be called multiple times for as many videos. Allowing id to accept either a string or an array of strings would be a convenient way to track multiple videos.

From a cursory glance at the code it seems like it would be fairly straightforward to add in a non-breaking way.