WL-3167 Allow outputting a <!DOCTYPE>
WL-3167 Add tests to isFiltered()

This means we can easily check we still filter the correct stuff.
WL-3167 Simplify the isFiltered() test.

Simplify the isFiltered() logic and make it more readable.
WL-3167 Switch wording from strict to standards.
Merge pull request #26 from ox-it/WL-3167

WL-3167 Allow adding of a <!DOCTYPE declaration