Added filter to $wp->query_vars['url']

In some cases the $wp->query_vars[‘url’] provides the wrong url for the
embed, for example, when .htaccess is used to generate custom urls. This causes a 'not found' on oemed links. 

Provided a filter so people can override the url if they need to.

We then made a filter function to convert the url to the right wordpress permalink so that the oembed links work.