[plt-scheme] Scribble changes

Mostly for those who use Scribble to generate documents other than
Scheme documentation...

The good news is that the back-end is now a little more configurable,
and it's extensible in a more composable way:

 * You can attach a `(css ,file) or `(tex ,file) style to a section,
   and then use the corresponding CSS/Tex definitions through a string
   style for an element, paragraph, or blockquote.

 * The "prefix", which you can specify through command-line flags when
   using the `scribble' executable, is now in two parts. The split
   mostly separates the Latex `documentclass' declaration (which you
   typically want to replace) from the Latex style macros (which you
   typically want to keep or override partially).

See the new Chapter 17 of the Scribble manual for more details.

The bad news is that the meaning of the `--prefix' flag for `scribble'
has changed. If you were using it before, then you'll need to adjust
your files and build command.