Add simplistic ⌃ F and ⌃ D bindings

I just wanted to put this in here before I forget. In default vim, [`CTRL-F`][^f] scrolls a full page, whereas [`CTRL-D`][^d] scrolls a half page (cursor remains at the same position with both). Their backward counterparts (⌃B and ⌃U respectively) operate the same way.

As a specific example, ⌃F scrolls 52 lines in my vim environment. ⌃D scrolls only 26.

This PR currently only adds ⌃F and ⌃D bindings to `core:page-down` and `core:page-up` respectively. It  doesn't add tests or modify the behavior of the ⌃D and ⌃U bindings.

If someone really wants the ⌃D and ⌃U functionality I can add it, otherwise it's not itching at me.

[^f]: http://vimhelp.appspot.com/scroll.txt.html#CTRL-F
[^d]: http://vimhelp.appspot.com/scroll.txt.html#CTRL-D