Fix a bug where the source node gets reclaimed by the GC

[eeror]

Let's say the user of the library initializes it like this:

const stream = await navigator.mediaDevices.getUserMedia({ audio: true })
const source = audioContext.createMediaStreamSource(stream)
const recorder = new WebAudioRecorder(source, recorderConfiguration)

At least in the newest version of Chrome, if source goes out of scope and gets reclaimed by the GC, the script processor will start receiving zeroes and record silence as a result.

If the user creates a source node just to pass it on to the recorder, it would be a leaky abstraction to require the user to keep a reference to it themselves. This pull request makes the library store a reference to the source node so the user wouldn't have to.

This will most likely fix #18, fix #28, and fix #29.