Figure out primary keys and update/alter

[simonw]

geojson-to-sqlite supports both --pk= and --alter options.

I'm not sure if --pk makes sense for Shapefiles - they appear to have a unijque ID property that I could use, but I'm not certain if it is guaranteed to be present. It's been present in the files I've looked at so far.

Adding tests that exercise --alter plus upserting data into existing tables would be worthwhile.