Support audio and video file URLs

We should support URLs for audio and video files as signals to create a media element based on native HTML playback devices, as we do in TimelineJS ([Audio](https://github.com/NUKnightLab/TimelineJS3/blob/master/src/js/media/types/Audio.js), [Video](https://github.com/NUKnightLab/TimelineJS3/blob/master/src/js/media/types/Video.js) )

