Popolo JSON export: add an option for output core_import_popolo can use

We're shortly going to remove the south-africa-popolo.json file (since
it's out of date) and instead point people towards the daily Popolo JSON
dumps.  However, the core_import_popolo command expects a particular
format (a dictionary with 'organizations' and 'persons' keys, the latter
having memberships inline), which core_export_to_popolo didn't generate.

This commit introduces a --pombola option, which will generate this
format.