Pass raw HTML from Reviewer through to lambdas

This is necessary now because the lambdas will be inspecting on-the-fly
output for <span class="cloze"></span> tags as the definitive marker of
such placeholders.
Cloze placeholder substitution

Introduces support for the cloze-related configuration settings,
allowing the user to intelligently swap out cloze placeholders with
either their hint, ellipses, or nothing at all.