Bash auto-completion for `./makesure`

[xonixx]

At least we need auto-completion of goal names in CLI.

• [x] make autocompletion work when -f given (autocompletion for make does this)
  • https://github.com/scop/bash-completion/blob/master/completions/make
• [x] add tests for auto-completion (expect)
• [x] understand where to install autocompletion script
• [x] update README

Useful links

• https://iridakos.com/programming/2018/03/01/bash-programmable-completion-tutorial
• https://www.gnu.org/software/bash/manual/html_node/Programmable-Completion-Builtins.html#Programmable-Completion-Builtins
• https://github.com/casey/just#shell-completion-scripts