Add zoom (a la optifine)

Add a "local zoom", just changing the FOV.

Option in the GUI for changing the keybind, "zoom fov" tweakable in minetest.conf.

Example images:

![Zoom out](http://exio4.com.ar/_/mt/zoom_out.png)
![Zoom in](http://exio4.com.ar/_/mt/zoom_in.png)