Tagging for exercise cloze

An old spec for DTBook markup from The Swedish Library of Talking Books and Braille ([Requirements for Text and Image Quality
and Markup with DTBook XML, Version: 2011-2](https://nlbdev.github.io/nordic-epub3-dtbook-migrator/resources/dtbook_markup_requirements_2011-2_v_1.docx) contains recommendations for markup of exercises and answers.

Tagging of exercises is covered in [Lists](https://daisy.github.io/ebraille/best-practices/tagging/#lists) with an [example](https://daisy.github.io/ebraille/best-practices/tagging/#example-a-multiple-choice-exercise) provided.

There doesn't seem to be any provisions for tagging of answers in eBraille however.

The old spec recommends to use a `span` with a specific class. The spec uses `answer` but a better class name would IMHO be `cloze` as this term seems to be a [commonly used](https://en.wikipedia.org/wiki/Cloze_test). There is even a stackoverflow answer on how to [create clozed text in HTML](https://stackoverflow.com/q/27370638).

The question remains whether a cloze is really that useful for electronic braille. In read-only mode it could provide some information but maybe a better solution would be to aim for real HTML forms where the cloze is a text field. This would then go more towards #86.