feat(clap_complete): add completion subcommand

Add clap_complete feature to duniter.

It takes the form of a new subcommand namely completion as follow:

USAGE:
    duniter completion --generator <GENERATOR>

OPTIONS:
    -g, --generator <GENERATOR>    [possible values: bash, elvish, fish, powershell, zsh]
    -h, --help                     Print help information
    -V, --version                  Print version information

This merge request is related to #37 (closed).