Obsoleted the `\preamble' command. Preamble text is now taken to be

any ordinary displayable paragraph(s) appearing before the first
chapter heading, meaning in particular that you can put lists, code
paragraphs etc in preambles. Of course, `\preamble' is still
supported for backwards compatibility, but it's now a zero-effect
paragraph marker.

git-svn-id: svn://svn.tartarus.org/sgt/halibut@3981 cda61777-01e9-0310-a592-d414129be87e