Implement shellcomp

Command line (aka tab) completion is popular in the Unix world as it helps typing speed, prevents typos and makes the shell more user-friendly. Impementing filename completion is easy. Implementing command-specific completion like git com is not. Completion scripts are different across Bash, Zsh and Fish. Time consuming to implement, sometimes out of date, hacky.

Shellcomp is a proposal for a shell completion protocol. Completion is implemented in the command about to be run: The shell run the command with a specific --tabcomplete '' option. The command responds with simple JSON structure that the shell will parse to perform completion or display help messages.