Show Textareas button selected with id not tag name

want other buttons!

(cherry picked from commit aec8ed02dae67bd14d907fd5720ef6c6dd73d536)
Make visual tests' "Show Textareas" way less hacky #test

Rather than swapping the CSS class of all textareas, and then having to
monkey-patch jQuery.fn.prepend so that when static math dynamically adds
textareas, their CSS class gets swapped too, just override the CSS of
textareas.
Fix selecting already selected static math

Only prepend textareaSpan if not already inserted, otherwise will detach
and re-attach, synchronously firing a blur event which in an immediately
subsequent event thread detaches the textareaSpan.

Deals with #216 for non-IE8; relies `return false` in mousedown handler
preventing textarea from being blurred, though, which apparently doesn't
work in IE<9.
Merge pull request #230 from mathquill/fix.select-static-math

Fix selecting already selected static math