add configuration to disable instrospection

[brunnogrillo]

In order to improve security I think we should be able to disable introspection via configuration.

GraphQL Introspection
GraphQL Cheat Sheet

[sangeetds]

Hi @brunnogrillo!
I agree with you on the security point. I have forked this repo and trying to maintain it. I thought it would be better if you could open the same PR at https://github.com/sangeetds/KGraphQL instead of me pushing your commit from here. Thanks!

[jeggy]

Hi @brunnogrillo.
Thanks for this new configuration