Add VolumeMeter package

[joggienl]

Add a new package that exposes a volume meter.
The output of this meter should be RMS value of some input (either audio or microphone).

Preferably this needs to be done by using an AnalyserNode but latest tests have shown that the function that exposes this info do exist but don't work.

Workaround could be using the ScriptProcessorNode (something like the onaudioprocess event) and or combine this with the AnalyserNode.

[kvdb]

I think you can close this one since you merged the AmplitudePlugin, right?

[joggienl]

Closing was correct.
References #341 and #335.