Adding new optional utility properties

[plauche]

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I ended up expanding this PR into three new optional properties:

• onStart - A function similar to onStop allowing for custom behavior when recording starts.
• customMediaStream - Instead of just passing in a deviceId I opted to allow the user to pass in their own MediaStream for total control over what is recorded.
• stopStreamsOnStop - The current default behavior is to stop the media streams when the recording stops. This flag preservers that behavior, but also allows the option to not stop streams when recording stops.

[DeltaCircuit]

Hey @plauche I know this PR has been here for more than a month. Had multiple production releases within the last 30 days at my work. So sorry for missing this.

This is tested with multiple cases, right? Can I take your word for it? I don't want to block this PR anymore because of my time constraints. Let me know if this can be merged. I'll merge it right away.

And a million thanks for taking your time and improving this library.

[no-1ne]

Hello passing customMediaStream is handy, any chance this pull request be revived

[royherma]

Please let's take care of this - customMediaStream is important!

[VinSpee]

I've resurrected this w/ the latest master in #92