Dispatches filters listed in an entry's filters.prop to scripts under
$wrt->{filter_dir}.

This is inefficient, in that it starts an entire process (at least) per
filtered entry.  On the other hand, it's a pretty clean abstraction, and
it means that filters could be implemented in whatever language.

Adds filters/ creation to wrt init.

- 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"