Implement is_wordpress_site for api discovery (#536)

* Implement simplified version of is_wordpress_site_discovery

* Implement fetch /wp-json as part of is-wordpress-site discovery

* Parse html in is_wordpress_site

* Introduce IsWordPressSiteParseHtmlResult

* Search link, script & style tags for wp-content & wp-includes

* Add is_wordpress_site helpers

* Add assertion to test_is_wordpress_site

* Only check the `href` of `link` and `src` of `script` for is_wordpress_site

Author: oguzkocer