Enhancement - Add optional 'continent' in geocoding namespace

[Codain]

I suggest to put this optional field containing the geographical continent name, following 7 continents model, in the properties.geocoding namespace i.e:

• Europe for Paris, France
• South America for Cayenne, France
• Antartica for -72.0129, 2.5284

This would especially be usefull for addresses where no country can be clearly defined, such as Antartica.

      // OPTIONAL. Geographical continent of the place, following 7 continents model (https://en.wikipedia.org/wiki/Continent#Number).
      "continent": "Europe",