Chasing the Platypus! - Added Margin Mirroring

What is happening in the development of Platypus.

Platypus is the free, comprehensive, and easy to use page-layout and typesetting system available at http://platypus.pz.org

Added Margin Mirroring

Oops, never ported this command. Added now as [mirrormargins]

Also added tests for [columnwidth] command, whose addition was completed yesterday