Add support for selecting horizontal or vertical navigation

For a 2-dimensional grid, I eventually would like to be able to have 2-d navigation support in knockout.selection. How  I'd like to solve that problem I'm not sure yet, see https://github.com/bramstein/knockout.selection/issues/78, but a first step would be to add support for horizontal navigation - currently we only have vertical navigation support, so horizontally ordered data sets aren't as intuitive to navigate through with the keyboard.