Handle for hex-bit holders.

This is a remix of the bit handle by frogfish_42.

Changes from the original:

• Improved customizer interface.
• Partially rewritten to make the code more readable.
• Socket hole for bit holder can now go deeper into the handle if necessary.
• Added a testing option to check the fit of the socket hole.

Edit: 2019-10-21 #1: Bugfix: Missing positioning of hex-bit socket hole.
Edit: 2019-10-21 #2: Made length of grip nozzle configurable.