Add support for \{,d,t}binom

Test Plan: `\binom xy^{\binom xy^{\binom xy}}` looks like something. `\dbinom` and `\tbinom` also seem to work.

Reviewers: emily

Reviewed By: emily

Subscribers: jessie

Differential Revision: http://phabricator.khanacademy.org/D13315

test/huxley/Huxleyfile.json

@@ -11,6 +11,12 @@
         "url": "http://localhost:7936/test/huxley/test.html?m=\\dfrac{a}{b}\\frac{a}{b}\\tfrac{a}{b}"
     },
 
+    {
+        "name": "BinomTest",
+        "screenSize": [1024, 768],
+        "url": "http://localhost:7936/test/huxley/test.html?m=\\dbinom{a}{b}\\tbinom{a}{b}^{\\binom{a}{b}+17}"
+    },
+
     {
         "name": "NestedFractions",
         "screenSize": [1024, 768],

test/huxley/Huxleyfolder/BinomTest.record.json

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