Add support for more phrasing elements in titles

[jeremyfelt]

In #68, we added the ability to use <span> inside of section and column titles.

We should discuss what other phrasing elements should be allowed in this scenario.

Possibilities would include: sup, sub, strong, em...

[natejacobson]

A nearly exhaustive list of inline elements is at

 https://developer.mozilla.org/en-US/docs/Web/HTML/Inline_elemente

Of that list, instead of trying to guess at a minimal possible set, I think we could be minimally restrictive and allow:

b, big, i, small, tt, abbr, acronym, cite, code, dfn, em, kbd, strong, samp, var, a, bdo, br, q, span, sub, sup, label, wbr, del, ins