Extract expand: Support JSON Arrays

[phaer]

Hi,

I needed to extract data in JSON Arrays to normalize data imports. I've quickly hacked the following together based on #241 which refers to #239 where you, @simonw, wrote:

Could this handle lists of objects too? That would be pretty amazing - if the column has a [{...}, {...}] list in it could turn that into a many-to-many.

They way this works in my work is that many-to-many relationships are created for anything that maps to an dictionary in a list, and many-to-one relations for everything else (assumed to be scalar values). Not sure what the best approach here would be? Are many-to-one relationships are at all useful here?

What do you think about this approach? I could try to add it to the cli interface and documentation if wanted.

Thanks for this awesome piece of software in any case! :sun_with_face: