Create a function
runOnKeys(func, code1, code2, ... code_n) that runs
func on simultaneous pressing of keys with codes
For instance, the code below shows
"W" are pressed together (in any language, with or without CapsLock)
runOnKeys( () => alert("Hello!"), "KeyQ", "KeyW" );
We should use two handlers:
pressed should keep currently pressed keys.
The first handler adds to it, while the second one removes from it. Every time on
keydown we check if we have enough keys pressed, and run the function if it is so.