Add a zsh completion function

[okapia]

This adds a completion function for zsh to complement the existing one for bash.

I've also modified Makefile.am here to install it in $prefix/share/zsh/site-functions which is the canonical place for zsh completion functions. For the bash completion, there is further logic in configure.ac but it is perhaps using pkg-config to query where the bash-completions project wants the files installed. I don't think anything similar would be applicable other than perhaps to make the installation optional. It does need to be installed with a name starting with an underscore for zsh's compinit to find it.

[jpmens]

Thank you for your contribution. I think the file _jo ought to be named along the lines of the bash completion (jo.bash). Care to rename the file to jo.zsh, please?

[okapia]

Very well. I considered doing this before even but the necessary automake changes to rename it on installation seem rather ugly. Seems to need a hook to rename it after already installing. Note that it does need to be installed with a name like _jo otherwise it won't get picked up by zsh's compinit. I've tweaked the comment in the header of the actual completion file accordingly.

[jpmens]

Merged, @okapia, thank you!