Feature request: add option for context factory function

[andrew0]

I'd like to be able to set the context of my GraphQL resolvers to ctx.state rather than just ctx. With many other GraphQL server implementations, I could do something like graphqlHTTP({ schema, context: (ctx) => ctx.state }), but this library only allows you to pass a static value as the context.

Since the default context is the Koa ctx, I could just use ctx.state in my resolvers, but I'd prefer to decouple the raw request details from my resolvers since it makes it easier to mock the context in tests.

[andrew0]

realizing now that the whole options can be a function...