Add append/remove subcommands to `bin/omero config`

This PR addresses https://trac.openmicroscopy.org.uk/ome/ticket/11201

Changes included are:
- addition of `bin/omero config append` and `bin/omero config remove` so that configuration using lists of strings can easily be manipulated, e.g.
 
 ```
bin/omero config append omero.web.apps webfigure
bin/omero config append omero.web.apps webtagging
…
bin/omero config remove omero.web.apps webfigure
```

- addition of corresponding unit tests under `test_prefs.py`

/cc @dpwrussell, @will-moore, @manics