it's not clear what happens if `HtmlQuery.attr(html, attr)` doesn't find the attr

[eahanson]

At the very least, add docs and examples.

• Maybe add attr? which returns true or false depending on whether the attr exists?
• Maybe if it doesn't find the attr, it returns nil instead of empty string?

[eahanson]

See https://developer.mozilla.org/en-US/docs/Glossary/Boolean/HTML