Write validator for deprecated fields

[stubailo]

Would be good to know what the message looks like, and if it works well!

[stubailo]

[stubailo]

Tweeted at Lee Byron to ask if there is an easy solution: https://twitter.com/stubailo/status/725396098584592384

[jnwng]

looks like we'd just need to integrate with graphql/graphql-js#389

[sdcooke]

I think this is as simple as master...sdcooke:deprecated to find the fields. I'm not sure how you would want to integrate this properly (e.g. making it an optional rule) and I haven't got my head around this codebase properly yet so someone else is probably much better placed to implement it fully.

Modifying the plugin directly I've got output along the lines of:

"The field X.X is deprecated. [deprecation message from schema]"

[stubailo]

Yeah I think a custom validation rule in the options would be good, or perhaps a separate option like "validateDeprecatedFields: true"

[koddsson]

This has been done in #93!