Allow playback of video to be muted (#41)

[whimboo]

Fixes issue #41.

For browsers / platforms where autoplay is blocked due to audio output, allow the start of the playback by just muting the audio channel.

[whimboo]

Is there anyone who could review this PR? Or is such an option not wanted?

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[whimboo]

I signed it!

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[whimboo]

@jiaqzhao thank you for merging the PR. Do you think that you could also cherry-pick for the 2019 branch?

[jiaqzhao]

Thanks for the PR. Yes, I will do that.

[whimboo]

Something I noticed is that it might be good to have the muted flag shown in the "menu" bar too, like next to Loop, or Log.