More granular endstop settings and define zprobe area parametrically

Hi Johann -
Here are two changes that I made to my Marlin fork that might be of interest to other Mini Kossel folks.

- allow the polarity of the _min and _max endstops to be set independently
- define the zprobe area parametrically in Configuration.h

I could also eventually see defining the deploy/retract coordinates in Configuration.h so that users don't need to dig into Marlin_main.cpp.

I'm sending this to you rather than ErikZalm because he doesn't have your latest changes integrated yet and I'd imagine that most Kossel people are working off of your fork.

Let me know what you think and if you have any questions. Thanks!
- joey