Support loose attribute value

Author: mantou132

grammar.js

@@ -124,7 +124,7 @@ module.exports = grammar({
 
     attribute_name: _ => /[^<>"'/=\s]+/,
 
-    attribute_value: _ => /[^<>"'=\s]+/,
+    attribute_value: _ => /[^<>"'=\s][^>\s]*/,
 
     // An entity can be named, numeric (decimal), or numeric (hexacecimal). The
     // longest entity name is 29 characters long, and the HTML spec says that