Add onEmpty command binding

[funbiscuit]

Currently on any user input something is called - either registered binding or general onCommand when given command is not registered. The only exception is if input empty (user pressed "enter" multiple times). In this case nothing is called.
Add extra optional command binding - onEmpty which will be called when entered command is empty (or contains only whitespace).

Initial request by @Jeff-RavenLabs in #18