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

[Troy Henderson]

Well the wiki is now setup.

http://metapost.tlhiv.org

Anonymous users can't edit pages, but anyone (for now) can create an
account.  I've themed it so that it doesn't look like everyone else's
wiki.  Anyone that would like to start adding content is free (and
encouraged) to do so.  I too plan on adding some stuff soon myself.

Troy