Add support for subscriptions

[vektah]

type Subscription {
    messageAdded(roomName: String!): Message!
}

will generate resolver methods that return a channel:

Subscription_messageAdded(ctx context.Context, roomName string) (<-chan Message, error)

these resolvers are expected to stop returning results when context is done.

I've also included an example apollo app that uses subscriptions:

Closes Subscriptions

[blasterpistol]

You are gem! Thank you for your amazing work! 🎉