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.

package.json

@@ -30,6 +30,7 @@
     "less": "~2.7.1",
     "morgan": "^1.7.0",
     "nomnom": "^1.8.1",
+    "object-assign": "^4.1.0",
     "pako": "1.0.4",
     "selenium-webdriver": "^2.48.2",
     "sri-toolbox": "^0.2.0",