Add `labels_only` arg to `FileHandler.printLabels`
Add option for only printing names of the labels

Add -n, --names-only optional arguments to the `list` subcommand.
Add support for short commands in Zsh completion

Add support for the completion of arguments when using a shorter
notation of subcommands: a[dd], ge[nerate], gu[i], h[elp], l[ist], r[m],
v[ersion].