add format.legend arguments to treemaps for value, density and manual.

[ellisp]

Key things to consider before accepting this:

• I've applied it to treemaps of type "value", "density" and "manual". I'm far from sure this is right (either too many or not enough), needs review / thinking.
• I've called the argument format.legend to make it consistent with position.legend. However, tmap has legend.format. @mtennekes to make a call on the right way to do it.
• I've made a single addition to the examples, just for the basic example (GNI, in my opinion fits best).
• I haven't changed the version or anything.
• anything else I haven't noticed.