Allow ⛩️📰 书社 to produce plain text

This requires adding _another_ build stage; the result of the
transformation step is output to `build/results`, which is then
processed again to create the `build/public` final result. In most
cases, this additional processing just produces a symlink. However,
when the root element is a special value, a derived file will be
produced.

The only special elements supported right now are `<书社:raw-text>`,
which outputs the raw text contents of the text nodes in the result
tree, and `<书社:base64-binary>`, which produces a binary file from the
base64 text contents determined using the same method.