Add option to select completion results by <Up>/<Down>

[hadashiA]

I need to select completion by Up / Down cursor keys.

Because:

1. This seems to be a standard behavior. e.g: browser's location bar, browser's input field, some editors and IDE code completion, etc.
2. In my environment, <C-n> / <C-p> is remapped to the cursor keys. This is more efficient than modifying the default behaviors of <C-n> / <C-p> for each application.