Adding leaflet-hash plugin

Regarding issue https://github.com/openstreetmap/iD/issues/1502, I also think that geolocated photos can help OSM mapping a lot. I use a lot of gelocated photos myself and I'm not a JOSM super user.

With this pull request users can easily recenter the map in iD by copy-pasting the hash given by the exif-extract location url.

Any suggestion on a safer way to link to leaf-hash plugin code welcome.