Add `ModifierKey` to keyboard.rs

[bvssvni]

Should support this since https://github.com/PistonDevelopers/input/issues/40 failed.

let mut modifier_key: ModifierKey = Default::default();
...
modifier_key.handle_input(&input);
if modifier_key == keyboard::CtrlShift {
     ...
}

This reacts to presses and releases of the modifier keys and keeps track of the state.