builder: Fix missing ProxyAdapter. Fix unsafe option docstring

re: d5857eac21aa3e4fc0a4bae514afd68189c17a30#commitcomment-7711092
Add regexstream module.
document: More abstractions. Put lxml concerns into document.htmlparse package
scraper: Follow document reader abstractions.
Fix up builder and convert to match document/scraper changes.
Update docs for document & scraper abstraction refactor changes.