Add -geometry option like x11vnc

[eric-oclock]

Hello,
thanks a lot for working on this great software.

i'd like to share only a part of my screen, for example 1024x768+50+0 ... i can do it with x11vnc on my linux but not with OSXvnc ... could you please add this feature ?

[stweil]

I am not sure when I'll have time to implement such a feature. So if someone has already implemented it, a pull request would be very welcome.

[stweil]

The command line option which x11vnc uses for clipping to a sub-region of the full display is -clip WxH+X+Y.