Add support for YAML comments (#305)

Author: ajdelaguila

lib/src/single_line_comments/parser/single_line_comments.dart

@@ -8,6 +8,7 @@ import 'package:highlight/languages/python.dart';
 import 'package:highlight/languages/scala.dart';
 import 'package:highlight/languages/typescript.dart';
 import 'package:highlight/languages/vhdl.dart';
+import 'package:highlight/languages/yaml.dart';
 
 class SingleLineComments {
   const SingleLineComments._();
@@ -22,6 +23,7 @@ class SingleLineComments {
     vhdl: [_hyphenMinuses],
     typescript: [_slashes],
     javascript: [_slashes],
+    yaml: [_hash],
   };
 
   static const _slashes = '//';