Idea: When the subcommand xyz (pijul xyz) does not exists, try to run pijul-xyz.

Motivation: @pmeunier has stated he wanted to reduce the number of commands of pijul, which is I think a reasonable goal. But in practice, one could want to be able to use additional, “unofficial” commands implemented thanks to libpijul.

For instance, one of my first contributions to pijul was the show-dependencies command, which does not exist anymore. I wonder if it could be possible to implement it back as a plugin, to install with cargo.

Done, thanks!