Add key filter to ignore keys from being registered by the KeyCombinator...

I made a small change to provide key filtering. I have yet to come up with a way to preserve default behaviour. Maybe you have advise? Basically I would like to preserve default behaviour of Tab (switch focus) and Enter (submit form).

....

It provides a new options field 'filterKeys' which takes an array of key
chars according to what KeyCombinator returns from its function
getKeyChar(keyCode).

eval_key_event() has been modified to return null in case the evaluated
key is in the options.filterKeys array.