Add support to toggle fullscreen using keyboard combination Alt + M

[michaeljqzq]

I use tablet which have keyboard, so add this feature

[xieby1]

Thanks for using this extension and for your contribution!

I think it is not necessary to change the code. There are two alternative ways to achieve your goal without changing code.

1. If you have a keyboard, you can press F11 to toggle fullscreen mode, which is supported by most browsers.
2. Go to chrome://extensions/shortcuts and set a keyboard shortcut for this extension.