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.

src/macros.js

@@ -0,0 +1,15 @@
+/**
+ * Predefined macros for KaTeX.
+ * This can be used to define some commands in terms of others.
+ */
+
+module.exports = {
+
+    //////////////////////////////////////////////////////////////////////
+    // amsmath.sty
+
+    // \def\overset#1#2{\binrel@{#2}\binrel@@{\mathop{\kern\z@#2}\limits^{#1}}}
+    "\\overset": "\\mathop{#2}\\limits^{#1}",
+    "\\underset": "\\mathop{#2}\\limits_{#1}",
+
+};