[metapost] improving MetaPost documentation (was: [mp-implementors] [mpman] appendix A: withcolor and withprescript/withpostscript)

[Troy Henderson]

> A wiki would obviously be entirely separate from the metapost
> distribution, so I'm not sure what this would mean.  But also I don't
> see that it's necessary to figure out such details before it even
> exists.  Just start the wiki and see how it develops :).

Done!  See

http://metapost.tlhiv.org

Now someone that is better at wiki's than I am should start making it
look NOT like other wiki's. :-)

Troy