Rename useHtmlParser2 option

This flag is used to control parsing behavior internally, but it is not
intended for use by consumers. Prefix the name with an underscore in
order to discourage users from relying on it.

lib/parse.js

@@ -37,7 +37,7 @@ exports.evaluate = function(content, options, isDocument) {
     content = content.toString();
 
   if (typeof content === 'string') {
-    var useHtmlParser2 = options.xmlMode || options.useHtmlParser2;
+    var useHtmlParser2 = options.xmlMode || options._useHtmlParser2;
 
     dom = useHtmlParser2 ? htmlparser.parseDOM(content, options) : parseWithParse5(content, isDocument);
   } else {

lib/utils.js

@@ -62,7 +62,7 @@ exports.domEach = function(cheerio, fn) {
  * @argument {Object} options - The parsing/rendering options
  */
 exports.cloneDom = function(dom, options) {
-  options = assign({}, options, { useHtmlParser2: true });
+  options = assign({}, options, { _useHtmlParser2: true });
 
   return parse(render(dom, options), options, false).children;
 };