Support for updating stream and teardown of previous resources

[kashifshamaz21]

Currently there is no way to start() hark monitoring. When the exposed function hark(stream, options) is triggered, it automatically starts the poll.
A better way would be to:

1. support an api to start the poll: hark.start()
2. on hark.stop(), clear the setTimeout, disconnect the analyser and free associated resources
3. Also, a way to update the media stream at runtime: hark.setStream() - this would first do hark.stop() to teardown the current existing pipeline, and then re-create it using the new stream.

[ibc]

Not sure why do you need so much sugar API. It's easier if you just call stop() on your Hark instance and create a new one with the new stream.