Support Exhibit Builder 3

See #5.

A proof-of-concept showing Neatline Time working with Exhibit Builder 3 (branch "next").

It's a very simple implementation, with the `getColumnPairs` on the Table (to enable `<select>` option generation) the only change to the core of the plugin.

Note, the layout.png image is a placeholder, currently just one of EB3's stock layout images.