Warn if options / ready function are passed to a previously created player instance 

There are questions fairly regularly on Stack Overflow that come down to confusion over using both `data-setup` and passing options and/or a ready function to `videojs()`.

This change logs a message with `vjs.log` if options or a ready function are passed as arguments, if a previously created player is returned. It also adds note in the setup doc.
Deep merging is hard. But otherwise, yes, it could happen.