- adds an example "flat" site with, for now, a single index in root for
    testing purposes
  - wrt-render-all handles default entry instead of just using "new"
  - EntryStore::all_renderable() correctly filters out paths ending in
    "/index" instead of "index", which if nothing else corrects a subtle
    bug that would have gotten in the way of naming something "windex"
  - WRT::handle() will return the contents of a root-level "index"

- 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.