New formula: pixz

Pixz (pronounced 'pixie') is a parallel, indexing version of XZ. This
formula successfully builds on OS X 10.8.3. It requires libarchive.
Added an option (--with-docs, hat tip to whoever did the git-cola
formula as well as the helpful folks in IRC) which will build and
install a manpage from the asciidoc source the author included.
Unfortunately, building the docs requires the installation of 2 other
packages, so I decided to keep it optional.

Closes #19698.

Signed-off-by: Jack Nagel <jacknagel@gmail.com>