internal: Be more forgiving about building `Taxonomy` objects. Add `setting_name`. The setting name of a taxonomy is whatever name is used to assign terms of that taxonomy to a page. If the taxonomy is "multiple", this is the name of the taxonomy. If not, it's the term name (a singular name).

*.pyc
venv/
build/lib
build/messages/_cache
dist/
piecrust.egg-info/
piecrust/__version__.py