feat: Adds support for custom keyboard shortcut behaviour

[scottlovegrove]

Unsure if you will approve of the implementation, but taken a stab at this anyway, mainly because I need a way to control when the keyboard shortcuts work (I have a scenario in my app that means some should be disabled).

Essentially this allows for developers to provide their own handlers for up/down/left/right keys being pressed. If provided, these will be called, and the internal defaults will not be called.

I've also implemented this in such a way that it can kick start what was raised in #107

[bruceharrison1984]

I'll have to noodle on this a bit and think through an implementation. I was more looking for a way to configure not only the keys pressed to change the month, but also a way to enable end users to configure swiping if they choose. This seems to allow an additional method to be called after an action is taken, but not configure what causes an action to be taken.

#78 kinda bleeds into this as well. This is somewhat supported if you override the header, and implement/inject code into the header to handle other scenarios.