Added support for setVolume command for WebAudio and AudioTag plugin

[DoomTay]

This will allow the use of the setVolume event, set by a type 7 controller MIDI event. This should fix #209.

[acosme]

@DoomTay is this working? can you show example code?

[DoomTay]

Here is a webpage demonstrating volume changes with a simple arrangement and a full-on song.

The song has a fadeout at the end. I'd have the player just skip to the end, but for whatever reason I couldn't get setting the currentTime working.