Add an `exwm-input-modifiers` setting

[Stebalien]

Right now, it's possible to instruct EXWM to intercept specific keys and key-combo prefixes via exwm-input-prefix-keys. However, there's no way to instruct EXWM to intercept all key combos including a specific modifier (e.g., super). This could be useful as most applications avoid binding super, leaving it for the window manager (in this case, EXWM).