Add config file

Hi Bryan,

This patch contains new code to move all possible configuration in a python-brlcad.cfg file, and load it from there.

Also it will try to find the brlcad installation based on the configuration, and on the PATH settings.

What it does: iterate over the brlcad versions configured (in that cfg file), and match it to the found installed version(s). Once a match is found, use that combination.

The code is ready in the meantime (added some more commits from my original pull request), and I tested it to work on linux.

I stripped out for the moment all Windows hacks, as I hope I will manage to make it work without them. I have a windows virtual machine to test on, coming next.

In any case, any parts of the python-brlcad.cfg file can be overridden in a per user ~/.python-brlcad.cfg - which makes customization somewhat easier. Once the code stabilizes, I will add instructions in the README about that.

Cheers,
Csaba