Enhancement: Rebind MUTE to M instead of 0 using FlxG's sound properties

[dinkley]

Please check for duplicates or similar issues before creating this issue.

There are duplicates, but I think this is the only one with an actual solution.

What is your suggestion, and why should it be implemented?

My suggestion is to rebind mute to M (or any other key) instead of 0, hopefully to prevent players from accidentally tapping 0 while playing. I did try to make a pull request but Git isn't playing well with my computer. I put the following code in MainMenuState, however that means that the new mute key only applies once the game actually reaches that state.

FlxG.sound.muteKeys = ["M"];