add pane switch commands

Implements the following pane switch commands.

`<c-w> h` - focus pane left
`<c-w> l` - focus pane right
`<c-w> j` - focus pane below
`<c-w> k` - focus pane above
`<c-w> <c-p>` - focus previous pane
`<c-w> <c-v>`/`<c-w> v` - create vertical split
`<c-w> <c-s>`/`<c-w> s` - create horizontal split

I manually implemented the focus switches so that the cursor would always be on screen after the switch.