Styling functions now use '+' to combine with tree grob
Test arguments to tree_style
Issues arising from the last round of work
The `+` method for adding to tree grobs is now extensible

Uses S3 methods, and some pokery to dispatch on the second argument
I.e., given tree + thing, dispatch on class of thing not of tree
(which will always be a tree grob)
Split the plotting file up.

Was getting a bit unweildly.  Hopefully this makes the similar bits
close together, but I'm not tied to the current split.
Fixes #47