Add Subscribe function to make easier to create Subscription Handlers

[racerxdl]

I added Subscribe function to Object / Fields Definitions so its easier to make GraphQL Subscription Handlers. I only added the field so it wouldn't break compatibility and if you don't use it, it wont make a difference.

So for example in https://github.com/racerxdl/go-subscription-handler I did:

var rootSubscriptions = graphql.ObjectConfig{
    Name: "RootSubscriptions",
    Fields: graphql.Fields{
        "serverTime": &graphql.Field{
            Type: graphql.String,
            Resolve: func(p graphql.ResolveParams) (interface{}, error) {
                err := subhandler.Subscribe(p.Context, "serverTime")

                if p.Source != nil {
                    // We received a event data
                    v, ok := p.Source.(map[string]interface{})
                    if ok && v["time"] != nil {
                        return v["time"], nil
                    }
                }

                // We didn't receive a event data, so resolve normally
                return time.Now().String(), err
            },
        },
    },
}

With that Subscribe field in the struct I could do:

var rootSubscriptions = graphql.ObjectConfig{
    Name: "RootSubscriptions",
    Fields: graphql.Fields{
        "serverTime": &graphql.Field{
            Type: graphql.String,
            Subscribe: func(p graphql.ResolveParams) error {
                return subhandler.Subscribe(p.Context, "serverTime")
            },
            Resolve: func(p graphql.ResolveParams) (interface{}, error) {
                if p.Source != nil {
                    // We received a event data
                    v, ok := p.Source.(map[string]interface{})
                    if ok && v["time"] != nil {
                        return v["time"], nil
                    }
                }

                // We didn't receive a event data, so resolve normally
                return time.Now().String(), err
            },
        },
    },
}

That should help with graphql-go#242

Ref: https://github.com/graphql-go/graphql/pulls