Support for GraphQL IDL schema file

[bessey]

Feature suggestion: the linter, unlike graphql-tool’s makeExecutableSchema, relies not on a GraphQL IDL file, but instead on the much longer introspection query file.

I version control my schema with each client, so there test suites can be run against a static target, it would be nice (tm) if I didn't need to do any legwork to convert my IDL to the JSON format.

[bessey]

Per @jnwng's reply on Slack:

i think the output of buildSchemaFromTypeDefinitions could be used to translate the IDL back to the schema we can parse with the graphql library (which we’re using under the hood)

[jnwng]

definitely, i like this since we can generate the executable schema. would happily accept a PR to this effect!

[stubailo]

Context: When this tool was first introduced, the IDL actually didn't exist! So adding it as an option alongside the introspection result would be awesome.