add completion on fish shell for `ml` command

[xdelaruelle]

When Modules is initialized on fish 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 fish shell completion for ml command, the init/fish_completion file has to be updated.

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.