[FEATURE REQUEST] Add addStream(mediaStream) method and 'stream' event for SimpleSignalClient

[OmarB97]

It would be great to have an addStream() method for SimpleSignalClient, similar to what you can do directly with simple-peer. Also a 'stream' event would be perfect so we can get a callback where we can do something directly with the mediaStream from the other peer.

function addStream(mediaStream) {
    ... 
)

.on('stream', (stream) => {
    ...
}