Add proper type inference by way of Subscription and SubscriptionWithHandler.

[parkerziegler]

This PR would close #70 and #72 and, in my opinion, should be the feature cap for v1 🎉 After @gugahoa's awesome GADT approach for useSubscription, I was really excited to port it over to the Subscription component. However, as this issue demonstrates (thanks for opening @gugahoa) the @react.component PPX has no idea what to do w/ locally abstract types. Bummer.

I don't expect to see a fix for that in the near future, so, in the interest of getting v1 out, I decided to expand the API surface to include Subscription and SubscriptionWithHandler. The former will infer Data(d) as the parsed result of the subscription, while the latter will infer it as the accumulator 'acc type inferred through use of the handler prop. I hate that we have to increase API surface to get proper type inference, but I also think it's small enough (and likely not used a ton in comparison to the hook) that I can live with it.

I added an example of using SubscriptionWithHandler to our subscription example to demonstrate usage. I had to create a new subscription type and resolver on the server since subscription-transport-ws has memory leek issues with multiple clients using the same subscription: apollographql/subscriptions-transport-ws#433

But hey, now we have some rad abstract art 😂

cc/ @Schmavery @gugahoa

[gugahoa]

LGTM 🚀