generic explain options v3

Here is an updated version of my "generic options for explain" patch.
Previous version here:

http://archives.postgresql.org/pgsql-hackers/2009-06/msg00866.php

This patch requires the "explain refactoring v4" patch, which you can
find here, to be applied first:

http://archives.postgresql.org/pgsql-hackers/2009-06/msg00865.php

In this version, I've taken the liberty of adding a "COSTS" option
which defaults to "ON", so that you can say: EXPLAIN (COSTS OFF) ...
to abolish display of the costs information, per my previous
suggestion. I was initially thinking of waiting to submit this as a
follow-on patch, but nobody seemed to object to the idea much, so I've
gone ahead and added it here. It remains to be seen whether someone
can develop a workable set of regression tests based on this
functionality, but it's pretty clear that it CAN'T be done without
this functionality, so this seems like a step in the right direction
at any rate.

The other major update in this patch is that it adds documentation. I
was not completely sure what the best way to document this was, so
it's very possible that what I've done here can be improved upon.

I will send updated versions of the "machine-readable explain output"
patches soon.

...Robert