Export to GeoJSON option

[wildintellect]

In discussing how users might want to access a spatially referenced Zotero library. Some users may want to simply export an existing Collection as a spatial layer. We should research options for providing this from Zotero.

Examples:

• The current leaflet map loops over Zotero items adding them one by one to the map, if the looped items are merged into a single geojson a user could download it.
• Users in QGIS might want to add someone else's Collection, based on it's web url to their map. This would be read-only, no save back to Zotero.