Added "muted" field to video object

[coenist]

Thank you very much for sharing this very useful plugin for adding videos to Sphinx. I just took the liberty to add a boolean attribute to load a muted video and hope you'll find this useful.

[12rambau]

superceeded by #13

Thanks for your contribution, I modernized this repo so I opened a bigger PR that englobe all the missing parameters including "muted"