- adds include_process() in WRT::Markup
- moves eval_perl() into WRT::Markup.  The way object instances are
  composed (and instance methods called) here still feels pretty janky,
  and this does nothing to help the situation.
- changes all existing $root_dir instances to $entry_dir; adds a $root_dir
  which represents the root of the archive (top-level folder containing
  the wrt.json file)

I'd be worried about the $root_dir change if I thought this module had a
single other user in the wild, but I strongly doubt that it does.  If I
am wrong and you are that user, I apologize sincerely in advance.

The include feature still has fairly unsettled semantics, but it gets the
whole thing much closer to a usable site generator.