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/SectionList.js

@@ -6,7 +6,9 @@ class SectionList {
   type = 'SectionList';
   
   constructor(data) {
-    this.target_id = data.targetId || null;
+    data.targetId 
+      && (this.target_id = data.targetId);
+      
     this.contents = Parser.parse(data.contents);
    
     if (data.continuations) {
@@ -17,7 +19,8 @@ class SectionList {
       }
     }
     
-    this.header = Parser.parse(data.header);
+    data.header && 
+      (data.header = Parser.parse(data.header));
   }
 }