Add HTML extension (#2082)

* Add html extension

* Fix flow error

* Update documentation

* Add tests

* Call buildA11yStrings for "html" node

* Throw ParseError when parsing \htmlData fails

* Improve documentation

* Add a screenshotter test

* Add dummy screenshot

* Update screenshots

contrib/render-a11y-string/render-a11y-string.js

@@ -655,6 +655,11 @@ const handleObject = (
             break;
         }
 
+        case "html": {
+            buildA11yStrings(tree.body, a11yStrings, atomType);
+            break;
+        }
+
         default:
             (tree.type: empty);
             throw new Error("KaTeX a11y un-recognized type: " + tree.type);