Added an option to allow a user to select different style next/prev keybindings

On OS X, ctrl+n / ctrl+p work fine, but on Windows those already have Chrome bindings. Personally, I like the ctrl+j / ctrl+k mappings which are similar to the Vim plugin CtrlP. I added a select menu in Options to choose one of the two styles. Up and Down will still always work, and I left ctrl+n / ctrl+p as the default style.

One other thing that I ran into, was that line 90 in options.js was throwing a JS error for me (even before my change). I commented it out here, but it is probably worth checking out.

Let me know if there is something you would like to see done differently.