add a 'listen' entrypoint to redirect to an audio only url

[eadmaster]

get_result was also changed a bit...

[jaimeMF]

Two things:

• Probably it should use bestaudio instead of m4a.
• It can be implemented in a simpler way (in my opinion):

@route_api('listen')
def listen():
    args = dict(request.args)
    args.setdefault('format', 'bestaudio')
    return redirect(url_for('.play', **args))

[eadmaster]

I've chosen mp4 because probably it is more compatible with older audio players that may lack opus support.

[jaimeMF]

I've chosen mp4 because probably it is more compatible with older audio players that may lack opus support.

If a player only supports m4a, you should specify the format.

[eadmaster]

Allrighty, i just have to add &format=m4a to my urls to get the audio track only...