Add mute button

A user said that they wanted to "pause to think". I've been using "mute tab" a lot myself, so I think a mute button for end-users is a great idea.

Consider using an `input` element with the `checkmark` type, and using CSS background images for the muted and unmuted (checked and unchecked) states:
https://stackoverflow.com/a/3772914/2234742