Bitholder Handle

Handle for hex-bit holders.

This is a remix of the [bit handle](/thing:3753399) by [frogfish_42](/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.