Add code for generating HTML

Test Plan: Ran unit tests. Looked at `\blue{\displaystyle \left(\dfrac{a^\sigma}{\sin \theta}\right\Updownarrow \intop_{1/2}^{z^z} \sum_{i=0}^\infty x \,dx}` in Chrome and saw the future in my eyes.

Reviewers: emily

Reviewed By: emily

Subscribers: jessie

Differential Revision: http://phabricator.khanacademy.org/D13154

katex.js

@@ -8,12 +8,18 @@ var process = function(toParse, baseNode) {
     utils.clearNode(baseNode);
 
     var tree = parseTree(toParse);
-    var node = buildTree(tree).toDOM();
+    var node = buildTree(tree).toNode();
 
     baseNode.appendChild(node);
 };
 
+var renderToString = function(toParse) {
+    var tree = parseTree(toParse);
+    return buildTree(tree).toMarkup();
+};
+
 module.exports = {
     process: process,
+    renderToString: renderToString,
     ParseError: ParseError
 };