Add compatibility with php7.2

Replace count() with strlen() from compatibility with php7.2.

Author: ralymov

src/PHPHtmlParser/Selector.php

@@ -168,8 +168,8 @@ protected function parseSelectorString($selector)
     protected function seek(array $nodes, array $rule, array $options)
     {
         // XPath index
-        if (count($rule['tag']) > 0 &&
-            count($rule['key']) > 0 &&
+        if (strlen($rule['tag']) > 0 &&
+            strlen($rule['key']) > 0 &&
             is_numeric($rule['key'])
         ) {
             $count = 0;