First support for md extensions; Package all needed dependencies in the repo

Source/node_modules/core-js/modules/es7.math.signbit.js

@@ -0,0 +1,7 @@
+// http://jfbastien.github.io/papers/Math.signbit.html
+var $export = require('./_export');
+
+$export($export.S, 'Math', { signbit: function signbit(x) {
+  // eslint-disable-next-line no-self-compare
+  return (x = +x) != x ? x : x == 0 ? 1 / x == Infinity : x > 0;
+} });