I think we now have plug-ins for heuristics and search algorithms. :-)

All heuristics (but not the other evaluators) are now (r4892) plugin-ized.

The search algorithms still need to be plugin-ized.

About documenting this on the wiki, I've decided against it for now since this
is all pretty simple from the existing examples and documentation needs to be
maintained. Maybe we should write some basic "How to add a new heuristic?"/"How
to add a new search algorithm?" tutorials later, but that should wait until
someone is motivated to do it and maybe also until the code has become more stable.

As of r4875, the plugin mechanism is now implemented for scalar evaluators
(heuristics). It is already used by the goal count heuristic, and the Makefile
has some notes on how to enable/disable plugins.

Still need to extend this to other heuristics, deal with search algorithms, and
document this in the wiki.

Wish: change the code to facilitate a "plug-in style" model for heuristics and
search algorithms, where new such components ("plugins") can be added to the
planner and/or removed from the planner by simply adding/removing them from the
Makefile.

This would also allow us to distribute additional heuristics etc. that we don't
feel comfortable about supporting as optional plugins, maybe in some kind of
"contrib" directory to make it clear that they are not part of the core planner.