[ Feature ] Allow for audio analyzer when webaudio engine used

[Cherryblue]

This gives options to the library user to get more information about the audio file being played by webaudio engine. For example, this allows the user to render a graphbar equalizer.

This allows to use the song analyzer via the following events :

• onswitchtowebaudio = function(trackPath, songAnalyser){}
• onplay = function(trackPath, webAudioEnabled, songAnalyser){}

You can modify the code however you feel it should be obviously :). It's working on my side, and I feel like it should be available for everyone.

[Cherryblue]

If you agree this is the correct way to do it, I could edit the readme to make known these added possibilities.