Add extensions function to createHandler

[bryanpyle]

Story

As a server, I want to add an extensions callback function similar to what's available in the express-graphql package options
so that I can add additional key-value metadata to each response. This is useful for capturing runtime and metrics for a given request.

Acceptance criteria

• The graphql-http createHandler function should have an optional extensions function for adding additional metadata to the GraphQL response as a key-value object. The result will be added to the "extensions" field in the resulting JSON. This is often a useful place to add development time metadata such as the runtime of a query or the amount of resources consumed. This may be an async function. The function is given one object as an argument: { document, variables, operationName, result, context }.