Implement up/down scrolling (CTRL-E, CTRL-Y, CTRL-U, CTRL-D)

[Kazark]

This implements the ability to scroll up and down in a window, with the proper bounds checking. Specify, four normal mode commands have been implemented:

• CTRL-Y (scroll up one line)
• CTRL-E (scroll down one line)
• CTRL-U (scroll up half a screen)
• CTRL-D (scroll down half a screen)

Due to F#'s composable (algebraic) data types and units of measure, this is thoroughly configurable just by bindings, without even any configuration values, i.e. we should not implement the &scroll option. You can just rebind one of these keys to a different command with a different number of lines or half screens, etc. Very very cool. Programmable rather than configurable.

@mjgpy3 Sorry for the humongous pull request! Do you want to field this one?

I had to do some technical changes to get this to work, specifically in the messaging system.

This is a solid basis for getting h, j, k and l working, which I will move to release three, and consider this the last thing for this release since it has dragged on a bit.

[mjgpy3]

Looking now!

[Kazark]

Thanks!

Good luck though, it's huge...

On the other hand, a lot of it is pretty clean, fresh stuff.

[mjgpy3]

Well, I looked through everything! I definitely don't understand everything going on here, but what I'm getting looks good.

Is this good to merge?

[Kazark]

Thanks for looking at it! I'm good with it when you are. Have you pulled it down and tried it out?

[mjgpy3]

Not yet! Will do, though.