Add tag CLI plugin work (gs-985)

This is a tag CLI plugin which makes tagging/tagset features accessible from the command line.

It currently adds the following tagging functionality at the OMERO CLI:

     # List tags (based upon criteria)
     bin/omero tag list ...

     # List tagsets (based upon criteria)
     bin/omero tag listsets ...

     # Create a new tag
     bin/omero tag create ...

     # Create a new tagset
     bin/omero tag createset ...

     # Import new tags from a JSON file
     bin/omero tag importjson ...

An additional linking functionality is in development. The currently proposed command format for this is:

     bin/omero tag link <object_type>:<object_id> <tag_annotation_id>

For the import function, current support is for a trivially simple JSON format. It might be decided (especially based upon any discussion here) that this is too inflexible and that we should switch to something else (such as XML or just a more detailed JSON). Comments and feedback on this are welcome.