URL source for audio toolbox

[kpreid]

/tools/audio-spectrum and friends can take audio from "user media" devices. I got a request to allow URLs for audio files/streams as well.

Details:

• Display the <audio> element so that play/pause/seek controls can be used.
• Optional play-through with volume control.
• Display errors accessing the URL (especially important as CORS will prevent some from working).
• Remember URL across page reloads.
• Maybe allow audio URL as a URL parameter as well, so it can be bookmarked/shared. (Maybe switch to URL parameters entirely instead of sessionStorage?)

This will require substantial refactoring of the UserMediaSelector code, but the previous design was kind of awkward and overengineered anyway.

[kpreid]

Commit ec44613ac0ebaadf183b66910969196a0de97a08

Besides some kludged-together bits, not done from the above list:

• Does not display an error on CORS failure.
• Does not put the selected source in a URL parameter.

But it works, and also supports local files.