GraphiQL Validation Rules? Plugin provided?

[acao]

Now that we have a prop for providing ValidationRules in GraphiQL, we can allow plugin developers or even users to configure how GraphiQL is validated. We could easily add the ability to register warnings as well.

there are many general purpose rules available already:
https://github.com/graphql/graphql-js/tree/master/src/validation/rules

these are the ones enabled by default:
https://github.com/graphql/graphql-js/blob/661ff1a6b591eea1e7a7e7c9e6b8b2dcfabf3bd7/src/validation/specifiedRules.js#L100

relay still has graphql-js compatible validation rules related to edges and nodes and other relay concepts

we can also allow plugin authors to define their own, very specific ones, if that ever makes sense to do? if anyone has examples of ways they would like to validate Gatsby queries for errors or warnings in GraphiQL that would be awesome!