Support Ctrl-N and Ctrl-P for navigating vertically

[smcgivern]

We are in the process of switching to Tribute from At.js. I don't think the team as a whole has any complaints, but I've found one issue: I can't use Control-N and Control-P to switch between items in the list.

This happens on the example page: https://zurb.github.io/tribute/example/

I wouldn't exactly class this as a bug, but these are standard keybindings for text inputs on macOS: https://support.apple.com/en-us/HT201236 They are also part of readline, so I'm used to using them pretty much everywhere: https://www.gnu.org/software/bash/manual/html_node/Commands-For-History.html

And At.js has supported them for a while: https://github.com/ichord/At.js/commit/ab9bd63d344e5bd67fbd5482caac03fb22bd1898

Would you be interested in accepting a patch to support this?