Add list placement to text and shield symbolizers from 2.1.0 onwards.

There's still the outstanding problem of how to describe the different placements, but since this enables advanced users to use a workaround, it's still useful. At the very least, this documents what these advanced users need to edit to get things working!

References (but doesn't fully fix) https://github.com/mapnik/mapnik-reference/issues/45

Enables a CartoCSS workaround - see
 https://github.com/mapbox/carto/issues/238#issuecomment-19673987