Split exportExtensions into exportDefault and exportNamespace plugins… (#6920)

* Split exportExtensions into two plugins in babylon
* rename proposal-export-default to proposal-export-default-from
* rename proposal-export-namespace to proposal-export-namespace-from

packages/babel-plugin-syntax-export-namespace-from/src/index.js

@@ -0,0 +1,7 @@
+export default function() {
+  return {
+    manipulateOptions(opts, parserOpts) {
+      parserOpts.plugins.push("exportNamespaceFrom");
+    },
+  };
+}