Setting mirroringMode  #49

Hi,

Setting the mirroringMode expects a session to already exist but crashes the app if the session has already started.

If I set sharedInstance.mirroringMode = .on before calling nextLevel.start() the mirroring mode is not taken into account.
https://github.com/NextLevel/NextLevel/blob/master/Sources/NextLevel.swift#L1758

If I set sharedInstance.mirroringMode = .on in the nextLevelSessionDidStart delegate the app crashes.