Export plugins example

[Chris Sperandio <sperandi eng usf edu>]

Not that anybody uses it (I will some day when I get around to writing
my dissertation :), but I'd like to update the metapost export plugin. 
I remember somewhere around the gtk2 updates (maybe prior) there was
some talk of new primitives being available to the plugins... or there
was basically a lot more that could be done with them than most are
using (my memory is shaky on this at best).  

At any rate, the plugin is broken as is, and while fixing it I thought
I'd give it an upgrade as well.  

Also, I'd like to write some simple doc [subsection] on using the
plugin--going from .dia->.mp->.eps.  It's not rocket science, but some
users who want tex output in their eps file may not know what to do
without it.

Thanks,
Chris