crash on non-JSON

[kraney]

If subscribed to a topic and that topic receives a message that's not proper JSON, this will crash Apollo.

The subscribe function needs a try/catch around JSON.parse().

[sethtippetts]

Having a similar issue. This library currently doesn't allow non-JSON payloads. I proposed in my PR using the NATS json field so the choice for serialization is left to the user. Hopefully my PR maintains backwards compatibility #5