Add new import option '-extra-tags'

[lonvia]

Implements #574.

The '-extra-tags' command line option takes a comma-separated list of OSM keys to be copied over from the Nominatim database.

When running updates, the user currently has to take care to include the '-extra-tags' again in the command line. We could probably scan the ES mapping instead to retrieve the configured list but I'll leave that for another PR.

[lonvia]

This is the wrong way to go about adding the mapping for the 'extra' field. We should be using dynamic templates to simply add the required mappings on the fly. With such a template it is possible to ensure that the field is not indexed.

[leonardehrenfried]

For this to work do you need the Nomatim extratags import style?
And do you need a specific version of Nominatim?

[lonvia]

Well, you need to make sure that the tags you are interested in have been previously imported into Nominatim. Using the extratags import style is one option. That style will ensure that most of the tags of an OSM object can be found in the extratags column. You can also start with one of the other styles and edit it to copy only the tags you need. Something like that:

{
    "keys" : ["cuisine", "takeaway", "website"],
    "values" : {
        "" : "extra"
    }
}

The new options works with all Nominatim versions that are usable with Photon.

[FaFre]

@lonvia this is a great feature! Helps me a lot, thank you very much :)