Builtin macros, macro arguments, overset and underset

* Ship predefined macros with the library, in macros.js.
* Allow macro arguments #1 and so on, with argument count deduced from string.
* Use these features to implement \overset and \underset, fixes #484.

test/katex-spec.js

@@ -2011,6 +2011,13 @@ describe("A macro expander", function() {
             "\\bar": "a",
         });
     });
+
+    it("should expand the \overset macro as expected", function() {
+        expect("\\overset?=").toParseLike("\\mathop{=}\\limits^{?}");
+        expect("\\overset{x=y}{\sqrt{ab}}")
+            .toParseLike("\\mathop{\sqrt{ab}}\\limits^{x=y}");
+        expect("\\overset {?} =").toParseLike("\\mathop{=}\\limits^{?}");
+    });
 });
 
 describe("A parser taking String objects", function() {