Add InputHandler trait, to allow client code to define custom keybindings

[rezural]

Add InputHandler trait which is settable on the pager struct, via set_input_handler(input_handler: Box)

Also add a DefaultInputHandler which handles input according to minus default, which is used by default.

The code which handles the internal minus Events, remains unchanged.

[AMythicDev]

I was thinking of something implementing something like this in minus. Thanks for working on it