build(aio): refactor dgeni packages

This is to tidy up the `author-packagse`, which currently duplicates a
lot of the configuration in the main packages. We need to
DRY this up so that we don't fall foul of a change in one being missed in
the other.

aio/tools/transforms/angular-base-package/readers/json.js

@@ -0,0 +1,19 @@
+/**
+ * Read in JSON files
+ */
+module.exports = function jsonFileReader() {
+  return {
+    name: 'jsonFileReader',
+    getDocs: function(fileInfo) {
+
+      // We return a single element array because content files only contain one document
+      return [{
+        docType: fileInfo.baseName + '-json',
+        data: JSON.parse(fileInfo.content),
+        template: 'json-doc.template.json',
+        id: fileInfo.baseName,
+        aliases: [fileInfo.baseName, fileInfo.relativePath]
+      }];
+    }
+  };
+};