Reuse Workers

• Initialize workers on Recorder instantiation.
• Add recorder.close() to tear down all the audio setup and workers, so it can be cleaned up by the browser
• Accept optional config parameter sourceNode which is an instance of MediaStreamAudioSourceNode
• Remove sourceNode parameter from recorder.start()
• Reuse the workers to avoid expensive setup when doing multiple recordings
• Resolve issue where audioContext needs to be resumed when suspended