Present captions on audio media

[DanOlson]

Description of what you did:

Caption support was already there, but was only wired up for video. This adds the existing text track support for audio media as well.

Addresses part of #747

If approved, this is ready to be merged

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[demiankatz]

@DanOlson, do you have any example manifests we can use to test/demonstrate this new functionality?

[DanOlson]

@demiankatz yes, here's one for an audio media item:

https://gist.githubusercontent.com/DanOlson/945c901a49ade5524bd1fb7fa920ceef/raw/e770069a5a77db5b20b0e7cc3326b1722e3b6f1f/interview-with-h-wyman-malmsten.json

and here's one for a video media item, for regression testing

https://gist.githubusercontent.com/DanOlson/e22318c3e6ea843784d5bc23d2f9b1ae/raw/c27df9dc0839ce0b071f80715928e1ed19da9064/assignment-for-tomorrow.json

The changes here assume the same structure for both audio and video manifests, where the canvas item's rendering property contains a Text object of format "text/vtt". No changes were made with regard to the expected structure of the manifest within MediaElementCenterPanel; I was just expanding on what existed previously for videos.

Please let me know if you need anything else. Thanks!