Add a more complex example

[kivutar]

This example is closer to a real life use of the library. It shows how to send data to subscribers as result of a mutation, as it is often the case in graphql. It also shows how to subscribe to a topic and broadcast only to the subscribers of this topic.

[kivutar]

With this last commit 7465fba963948454ec8417150e83f007596170da, we need this PR merged: https://github.com/graphql-go/handler/pull/33