Add support for \phantom

Summary:
Using \phantom with non-phantom math in Perseus doesn't render to be the
same size because \phantom uses MathJax and the non-phantom math uses KaTeX.
Implementing \phantom in KaTeX should solve this alignment issue.

Test Plan:
[x] write (and run) unit tests
[x] create (and run) screenshotter tests

Reviewers: emily

Reviewed By: emily

Differential Revision: https://phabricator.khanacademy.org/D16720

src/functions.js

@@ -164,6 +164,23 @@ var functions = {
                 type: "katex"
             };
         }
+    },
+
+    "\\phantom": {
+        numArgs: 1,
+        handler: function(func, body) {
+            var inner;
+            if (body.type === "ordgroup") {
+                inner = body.value;
+            } else {
+                inner = [body];
+            }
+
+            return {
+                type: "phantom",
+                value: inner
+            };
+        }
     }
 };