Option to normalize {audio-media} or {audio-context} volume against {audio} about memento

[ripose-jp]

Normalizing {audio-media} and {audio-context} is easy. Normalizing it to {audio} is not because those are remote files and they may not even exist.

I'd be willing to add an option to normalize to a user specified dB value, but not so much to {audio} specifically.

[Calvin-Xu]

That seems great too. JapanesePod101's audio loudness seems to vary from -14 to -25 (integrated) LUFS across my samples. -19.776 is the mean value I found if that may be a data point for determining the default.

[ripose-jp]

Thanks for getting the average volume of JapanesePod's audio clips. I've added an option to normalize audio and the default value is -20 dB. Everything sounds about as loud as JapanesePod, so it seems like a good default.

The changes are up on master.