refactor: Get the taxonomy support back to a functional state. There's now a taxonomy content source that wraps another normal content source like a blog posts' source. It works in tandem with a taxonomy content pipeline that will do the heavy lifting of figuring out what kind of terms exist and need to be baked.

*.pyc
*.egg-info
.cache
__pycache__
venv
tags
bower_components
node_modules
build/lib
util/messages/_cache
util/messages/_counter
dist
docs/_cache
docs/_counter
foodtruck/static
piecrust.egg-info
piecrust/__version__.py
.ropeproject