Add support for key modifiers (toggles)

[elgopher]

Some keys are like toggles - caps lock and num lk. In order the change the state of the modifier user has to press and release the button. Game might use this information somehow.

At the moment the keyboard package does not provide such information. Maybe we can add an API for checking the state of mentioned buttons. Something like:

keyboard.CapsLockOn()
keyboard.NumLockOn()