message: Add <ol start="..."> to allowed attributes list
  Fixes vector-im/riot-web#3273
diff: @@ -96,6 +96,7 @@ var sanitizeHtmlParams = {
  // We don't currently allow img itself by default, but this
  // would make sense if we did
  img: ['src'],
  + ol: ['start'],
  },
  // Lots of these won't come up by default because we don't allow them
  selfClosing: ['img', 'br', 'hr', 'area', 'base', 'basefont', 'input', 'link', 'meta'],
  