CLI: get valid --format values via autodiscover

[mtmail]

When adding a new format to nominatim/api/v1/format.py one also needed to add the format into the CLI. Now the format gets found automatically:

Before

nominatim search -h
nominatim reverse -h
nominatim lookup -h
  --format {xml,json,jsonv2,geojson,geocodejson,debug,myformat}

Now

  --format {xml,geojson,geocodejson,json,jsonv2,myformat,debug}

[lonvia]

Nice!