Allow user to access stream object -- can be used e.g. to stop recording.

[allen-edia]

Hi, this is intended to address #7.

The stream object is exposed to the application code.

The application code can then stop the stream with code like:

        const { stream, } = <your webcam instance>
        if (!stream) return console.error("No stream object, can't stop!")
        stream.getTracks().forEach(track => track.stop())

[nadeesha]

@amw, is there any way to get this merged? This is a trivial change that alleviates some crucial pain points. (Like turning off the webcam) Thanks!

[inakiarroyo]

@amw I am having the same issue of #7 and this PR solves the problem, could it be merged? Thanks!