handle sites with an index file in the root
- adds an example "flat" site with, for now, a single index in root for
- 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"