Allow import commonmark from 'commonmark'.

We still allow the `import * as commonmark from 'commonmark'`,
introduced in 0.29.2, as well.  But perhaps this should be
deprecated?

Closes #195.

lib/index.js

@@ -10,9 +10,20 @@
 // var parser = new commonmark.Parser();
 // var renderer = new commonmark.HtmlRenderer();
 // console.log(renderer.render(parser.parse('Hello *world*')));
+//
+// With ES6 modules:
+//
+// import commonmark from 'commonmark';
+//
+// or
+//
+// import * as commonmark from 'commonmark';
+
+import Node from "./node.js";
+import Parser from "./blocks.js";
+import Renderer from "./render/renderer.js";
+import HtmlRenderer from "./render/html.js";
+import XmlRenderer from "./render/xml.js";
 
-export { default as Node } from "./node.js";
-export { default as Parser } from "./blocks.js";
-export { default as Renderer } from "./render/renderer.js";
-export { default as HtmlRenderer } from "./render/html.js";
-export { default as XmlRenderer } from "./render/xml.js";
+export { Node, Parser, Renderer, HtmlRenderer, XmlRenderer };
+export default { Node, Parser, Renderer, HtmlRenderer, XmlRenderer };