New feature: fullscreen mode

Hi,

First of all, I want to thank you for monsterwm. I've been using it for some days now, and it's my favourite window manager so far (after years of using Ion, StumpWM and other tiling window managers) ☺

There's a feature I like which is on StumpWM but isn't on monsterwm: the ability to move to a fullscreen mode and return from it to the previous state with a single key binding. You can mimic this by using the monocle layout and then returning to the layout you used before, but this is not as easy as hitting `M-f` and then `M-f` again when you're done (and there's the issue of manually turning the bar off and on).
I use this feature a lot: when I need to do something in a window that requires the whole screen, I just pop in fullscreen mode, do whatever I need to, and than returning to the usual tiling layout.
This is very convenient IMHO, and I'd like to see this feature incorporated into the main branch of monsterwm, or at least given as a patch/branch on your repo.

This is my implementation of it, with some additional editing of the documentation.

Have a wonderful day,
Júda