For each table we want to know the columns, their order and what type they are.

I'm going to break with SQLite defaults a little on this one and allow datasette to define additional types - to start with just a geometry type for columns that are detected as SpatiaLite geometries.

Possible JSON design:

"columns": [{
    "name": "title",
    "type": "text"
}, ...]

Refs #276