Expose units in the JSON API?

[russss]

From #203: it would be nice for the JSON API to (optionally) return columns rendered with units in them - if, for example, you're consuming the JSON to render the rows on a map.

I'm not entirely sure how useful this will be though - at the moment my map queries are custom SQL queries (a few have joins in, the rest might be fetching large amounts of data so it makes sense to limit columns fetched). Perhaps the SQL function is a better approach in general.