cli: Stricter and safer config parsing.
cli: Stricter and safer config parsing.

Resolves #432.

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