Add experimental @live directive to subscribe to query

[KyleAMathews]

In a conversation with @dschafer he suggested using a @live directive as a way to tell the graphql server to create a subscription around a query.

Per #41 support for this would need to be added to graphql-js itself?

[leebyron]

#215 adds the ability to provide custom directives to a schema, but of course you'll still have to interpret those directives yourself :)

[KyleAMathews]

👍🏻 thanks!