plugins.cdnbg: rewrite plugin
plugins.cdnbg: rewrite plugin

- don't use suffix operators in pluginmatcher regex
- don't define validation schema and regexes as class attributes
- replace itertags with xpath query
- make validation schemas more legible by adding _find_url util method

bastimeyer/streamlink,streamlink/streamlink,bastimeyer/streamlink,chhe/streamlink,chhe/streamlink,streamlink/streamlink,gravyboat/streamlink,gravyboat/streamlink