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 `blur`-ed, though, which apparently doesn't
work in IE<9.