CSV plugin: support JSON encoded geometries

[springmeyer]

In addition to detecting lon/lat columns of point coordinates, the CSV plugin can also currently auto-detect any column named wkt and attempts to parse as WKT encoded geometries.

We can do the same for GeoJSON.

The tricky part will be supporting various json row quoting flavors.

We should support at least these forms:

"{\"type\":\"Point\",\"coordinates\":[30.0,10.0]}" // escaped "

'{"type":"Point","coordinates":[30.0,10.0]}' // single quotes no need for escaping "

"{""type"":""Point"",""coordinates"":[30.0,10.0]}" // filebakery.com style ""

Downstream issue: mapbox/tilemill#1618