withCredentials

This adds a plugin option called `withCredentials` which enables `withCredentials` on the XHR requests for the manifest and ts files.
The XHR helper also received a `responseType` option which was needed for the segmentXHR migration.
Segment XHR was migrated to use the new xhr helper.
Also, we updated the check for options to make sure it has a url in it to use. Otherwise, it'll continue to try and get the source urls from other places.