Add autocompletion

[MartinHelmut]

Using Litr should be easy, adding autocompletion therefore a must. Here some random resources for a start:

• https://www.gnu.org/software/bash/manual/bash.html#Programmable-Completion
• https://github.com/scop/bash-completion
• https://askubuntu.com/questions/68175/how-to-create-script-with-auto-complete/345150#345150
• Help text for Litr #25

[MartinHelmut]

Daily usage shows this is even more important 😅