Conditional charspan generation

* Now, charspans are only generated if the story passages contain either
".char" or "[data-char", which should be enough to sniff for
charspan-selecting CSS. I didn't think charspan generation was all that
much of a performance hit, but OK.
* Reverted the <br> removal, on the basis that using only white-space:
pre-line messes with text copied from passages. Hrm.
* Added the "hidden" HTML attribute to the storeArea div.