[ADD] Users can manage their stream

[marcsances]

AS A guild user
I WANT TO add my stream using !addstream command
SO THAT my stream is notified in the live channel if I have the appropiate roles

AC1 - there is a !addstream command to add user stream
AC2 - the user stream is replaced if it had already a stream (user is notified of that)
AC3 - RoleLive makes sure the Twitch stream actually exists before recording it into the database
AC4 - Users can only add a stream if allowed by guild configuration
AC5 - there is a !removestream command to remove the added stream if any