Move CLI parsing to clap.rs

[awidegreen]

All subcommands have been converted to use clap.

completions sub-command has been added to automatically generate shell completions for bash, zsh and fish

make list sub-command default action when no subcmd is specific from cli.

• add shell completions for bash, fish and zsh to contrib/ directory
• update README with regards to shell completions

[awidegreen]

I've pushed the rebased version of this merge request.

[maralla]

Thanks!