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/rendering/trimBlankLines.js

@@ -0,0 +1,15 @@
+module.exports = function() {
+  return {
+    name: 'trimBlankLines',
+    process: function(str) {
+      var lines = str.split(/\r?\n/);
+      while (lines.length && (lines[0].trim() === '')) {
+        lines.shift();
+      }
+      while (lines.length && (lines[lines.length - 1].trim() === '')) {
+        lines.pop();
+      }
+      return lines.join('\n');
+    }
+  };
+};