feat(plugin-helper): lowercase resulting tag names (#42)

packages/bbob-parser/test/parse.test.js

@@ -59,7 +59,7 @@ describe('Parser', () => {
       'Foo',
       ' ',
       {
-        tag: 'Bar',
+        tag: 'bar',
         attrs: {},
         content: []
       },

packages/bbob-plugin-helper/src/TagNode.js

@@ -3,7 +3,7 @@ import { getNodeLength, appendToNode } from './index';
 
 class TagNode {
   constructor(tag, attrs, content) {
-    this.tag = tag;
+    this.tag = tag.toLowerCase();
     this.attrs = attrs;
     this.content = [].concat(content);
   }