Add square roots (\sqrt)

Summary:
Follow the TeXbook instructions on how to construct square roots. Using
makeCustomSizedDelim, this becomes nearly trivial.

Test Plan:
 - Make sure normal tests work
 - Make sure the new huxley test looks good, and other huxley tests haven't changed.

Reviewers: alpert

Reviewed By: alpert

Differential Revision: http://phabricator.khanacademy.org/D12918

test/huxley/Sqrt.hux/record.json

@@ -0,0 +1,5 @@
+[
+  {
+    "action": "screenshot"
+  }
+]