Rename typesetting div class to .CodeMirror-MathJax-typesetting

Use case:
I've discovered MJ measuring & rendering from the typesetting div
(commit d77f0fc, #33)
somehow made math small in Mathdown, due to mathdown.css having:
.CodeMirror { font-family: "lining_numerals_charter", "Georgia", ... }
If I drop out the (nearly empty) first font "lining_numerals_charter",
it works out.
=> I'm just gonna style the typesetting div without this font,
   so I want it to have a better "public class name".

render-math.js

@@ -230,7 +230,7 @@ CodeMirror.hookMath = function(editor, MathJax) {
   typesettingDiv.style.height = 0;
   typesettingDiv.style.overflow = "hidden";
   typesettingDiv.style.visibility = "hidden";
-  typesettingDiv.className = "CodeMirror-MathJax";
+  typesettingDiv.className = "CodeMirror-MathJax-typesetting";
   editor.getWrapperElement().appendChild(typesettingDiv);
 
   // MathJax is much faster when typesetting many formulas at once.