Expose user-defined meta-information via introspection API in form of directives

This is a copy of https://github.com/graphql/graphql-spec/issues/300. I'm opening this to check if there's any interest in implementing this feature even without the spec supporting it.

The reason is that this feature would be quite useful to have and two major implementations are already supporting it or working to implement it (Java and .NET, see linked issue).