Add focus-monitor-next command

[augustebaum]

I only have two monitors (my laptop and one external monitor), so all the directional focus-workspace-* commands aren't very useful for me; however, at present I have to set two separate commands for moving between my monitors (focus-workspace-up and focus-workspace-down), whereas I would rather have just one focus-monitor-next command that wraps around the monitor list.
My need would also be served by focus-workspace-down (or -up) wrapping around.