addKeyBinding( binding, callback ) {

		if( typeof binding === 'object' && binding.keyCode ) {
			this.bindings[binding.keyCode] = {
				callback: callback,
				key: binding.key,
				description: binding.description
			};
		}
		else {
			this.bindings[binding] = {
				callback: callback,
				key: null,
				description: null
			};
		}

	}
Add a custom key binding with optional description to
be added to the help screen.