Allow underscore in attribute and element names (OWASP#127)

Author: Chuck Dumont

src/main/java/org/owasp/html/HtmlStreamRenderer.java

@@ -364,6 +364,7 @@ static boolean isValidHtmlName(String name) {
  if (i == 0 || i + 1 == n) { return false; }
  break;
  case '-':
+ case '_':
  if (i == 0 || i + 1 == n) { return false; }
  break;
  default: