plugins.letontv: Make use of the validation API.
plugins.letontv: Make use of the validation API.

Also add support for leton.tv/broadcast/<channel> URLs.

fishscene/streamlink,back-to/streamlink,Saturn/livestreamer,Masaz-/livestreamer,chhe/streamlink,streamlink/streamlink,derrod/livestreamer,mmetak/streamlink,Feverqwe/livestreamer,wolftankk/livestreamer,Saturn/livestreamer,caorong/livestreamer,chhe/streamlink,javiercantero/streamlink,programming086/livestreamer,melmorabity/streamlink,ethanhlc/streamlink,javiercantero/streamlink,chrippa/livestreamer,hmit/livestreamer,sbstp/streamlink,gravyboat/streamlink,beardypig/streamlink,wlerin/streamlink,streamlink/streamlink,flijloku/livestreamer,gravyboat/streamlink,Masaz-/livestreamer,chhe/livestreamer,okaywit/livestreamer,Klaudit/livestreamer,fishscene/streamlink,wlerin/streamlink,intact/livestreamer,Klaudit/livestreamer,lyhiving/livestreamer,back-to/streamlink,caorong/livestreamer,jtsymon/livestreamer,chhe/livestreamer,ethanhlc/streamlink,gtmanfred/livestreamer,melmorabity/streamlink,Feverqwe/livestreamer,programming086/livestreamer,bastimeyer/streamlink,flijloku/livestreamer,sbstp/streamlink,charmander/livestreamer,okaywit/livestreamer,blxd/livestreamer,lyhiving/livestreamer,chrippa/livestreamer,mmetak/streamlink,gtmanfred/livestreamer,Dobatymo/livestreamer,wolftankk/livestreamer,blxd/livestreamer,charmander/livestreamer,bastimeyer/streamlink,hmit/livestreamer,Dobatymo/livestreamer,intact/livestreamer,beardypig/streamlink,derrod/livestreamer,jtsymon/livestreamer