Adding configurable keybindings for up/down arrows

[gwendolyngoetz]

This PR changes the KeyArrowUp and KeyArrowDown to pull from the keybindings config. Up and down arrows are still the primary keybindings for moving between items on a page, but J and K are now alternates (to act like Vim).

This partially contributes to the request in #129.

Let me know if you need me to make any changes. Thanks!