Recurseively add custom paths to system.path

This PR changes 00-imports.py to allow for a simple way to add non-system directories to your python path.  Essentially, you build a list of full system paths (`PYTHON_MODULES_DIR_LIST`) and the loop uses os.walk() to recurse through the directories, adding all non-package, non-.git directories to the system path.  This seems to the be appropriate place for this code (at the very start), so that you can import modules in later parts of the startup files.

If `PYTHON_MODULES_DIR_LIST` is empty (the default) this code is completely skipped.

I've found this pretty useful, but feel free to ignore it!  Cheers...