Make debug-mode toggleable on keydown

[andrewpbray]

Options in yaml would be debug-mode: {true, false} with default to false. if true, could be toggled off and back on.

• Lua filter would check meta and if toggle, write option into <meta>
• In JS, if <meta>, set-up event listener for keydown that will add, remove debug class.

[jimjam-slam]

The only thing we need to watch out for/test with hot keys is ensuring they don't consume events intended for other things (like preventing text from being entered in a text box)!