Add Section and Article to DefaultBlockRenderMap (#2212)

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**Summary**

Fixes #2074
Updated the default block render map to have the section and article tags.
Pull Request resolved: #2212

Reviewed By: claudiopro

Differential Revision: D18246366

Pulled By: mrkev

fbshipit-source-id: 144f7721d92d6efcbb5c969ba992841eb9bd5f15

Author: Tarun047

src/model/constants/DraftBlockType.js

@@ -27,7 +27,9 @@ export type CoreDraftBlockType =
   | 'ordered-list-item'
   | 'blockquote'
   | 'code-block'
-  | 'atomic';
+  | 'atomic'
+  | 'section'
+  | 'article';
 
 /**
  * User defined types can be of any valid string.

src/model/immutable/DefaultDraftBlockRenderMap.js

@@ -47,6 +47,12 @@ const DefaultDraftBlockRenderMap: DefaultCoreDraftBlockRenderMap = Map({
   'header-six': {
     element: 'h6',
   },
+  section: {
+    element: 'section',
+  },
+  article: {
+    element: 'article',
+  },
   'unordered-list-item': {
     element: 'li',
     wrapper: UL_WRAP,