feat: tidy things up and implement more renderers

- Finished Library parser
- Fixed search continuations
- Improved channel parser
- Improved playlist parser
- Added support for posts of type poll
- Improved History parser
- Removed redundant code

lib/parser/contents/classes/ExpandedShelfContents.js

@@ -7,7 +7,11 @@ class ExpandedShelfContents {
   
   constructor(data) {
     this.items = Parser.parse(data.items);
-    this.contents = this.items; // XXX: alias for consistency
+  }
+  
+  // XXX: alias for consistency
+  get contents() {
+    return this.items;
   }
 }