Intuitive window splitting keybindings

This adds some alternative window splitting bindings that although they are not standard, are totally intuitive and easy to remember. No more do you have to wonder, "what is a vertical split? Is it the orientation of the split line or the stacked windows."

The alternatives are <kbd>w -</kbd> for creating a split line that runs horizontally, like a `-`. 
And <kbd>SPC w /</kbd> for creating a split where the line is vertical, like a `/`, it isn't a pipe because that's hard to type.

I've used bindings like these in tmux for a long time and have loved them, even though I use it maybe once every 3 months I always remember the splitting bindings and which kind of split I will get.