Missing autocompletion script

[hhoeflin]

Themite should support autocompletion script. This should work without calling the original python library (as this may be slow when dependencies on certain things exist).

• https://iridakos.com/programming/2018/03/01/bash-programmable-completion-tutorial
• https://www.endpointdev.com/blog/2016/06/adding-bash-completion-to-python-script/
• https://github.com/tiangolo/typer/blob/master/typer/_completion_shared.py