Allows SNICallback instead of hardcoded key/cert
Allows SNICallback instead of hardcoded key/cert

This updates the previous PR https://github.com/peers/peerjs-server/pull/219/ with the current code. It solves #209, allowing ssl options to use SNICallback instead of only a hardcoded key/cert pair. @afrokick I saw you accepted a few PRs this week, any chance this once can be included too? Thanks
peers/peerjs-server,peers/peerjs-server