Add function to remove canvas elements

[reesvarney]

I've been using a slightly modified version of this in my own project which has a lot of streams being mixed together/ dropped over periods of time. This causes issues as after calling releaseStreams() there is still a canvas element left in the DOM which can cause a lot of build-up over time.

I propose that a basic end() function is added which serves the same function as releaseStreams but also removes the mixer canvas element. This should be as simple as adding the following code:

this.end = function(){
    this.releaseStreams();
    canvas.parentNode.removeChild(canvas);
}

[reesvarney]

I would add this and PR myself but I don't have much experience with the typescript workflow.