empty values for option names

[debruine]

Give all options a default non-empty name if the name isn't set to "answer" so that the resulting HTML is technically correct. Options without values trigger an HTML warning that makes epubs impossible to render.

Triggers an error when rendering as epub:

mcq(c(answer = "TRUE", "FALSE"))

Does not trigger an error:

mcq(c(answer = "TRUE", x="FALSE"))