Feature: add speed (playback rate) control/selector

[aminamos]

Allow users to set their desired playback rate, provided a set number of options like:

• 0.25
• 0.5
• 0.75
• 1.0
• 1.25
• 1.5
• 1.75
• 2

[gkatsev]

This is available now! You can set playbackrates attribute as a space-separate list like so:

<mux-player playbackrates="1 2 3"></mux-player>