add completion on zsh shell for `ml` command

[xdelaruelle]

When Modules is initialized on zsh shell both module and ml commands are defined. Shell completion is provided for module command but it is currently missing for the ml command.

To add zsh shell completion for ml command, a init/zsh-functions/_ml.in file has to be created.

Shell completion for the ml command is available for bash shell. So looking at init/bash_completion.in file may provide some hints on the completion code to achieve. Also the zsh completion for module command in init/zsh-functions/_module.in could also help.