find: Fix the `find` command, add more options. The `--endpoint` option doesn't make any sense anymore since page sources could have non-file-system endpoints. This was replaced with `--name` to match source names instead. Also added `--include-theme`, with filtering out theme pages by default, and `--exact` to explicitely match the given pattern, with matching anything that contains the pattern by default.

syntax: glob
*.pyc
venv
tags
build/lib
build/messages/_cache
build/messages/_counter
dist
piecrust.egg-info
piecrust/__version__.py