Auto-completion should suggest target names

[fbecart]

zinoma already provides auto-completion for the static flags and options of the command line.

However, available target names depend on the project being built. It would be very helpful if the auto-completion could help on the targets of the current project. This would be either the project of the current directory, or of the project specified by the option -p, --project <PROJECT_DIR>.

The best would be to have this solved directly through clap. The feature doesn't exist yet, and the issue can be tracked at clap-rs/clap#1910

Alternatively, we could add a new flag: --complete-targets. If this flag is provided, zinoma would return a list of the available targets for the selected project. This command could be called from our completion scripts. Unfortunately, this solution would probably mean writing our completion scripts by hand (as opposed to having them generated by clap, which is currently the case).