Support custom document loading

[williambailey]

It would be nice to be able to provide a function to support custom document loading. Being able to extend/augment/replace the new Source(...), parse(source), validateFn(...) process would make implementing functionality like persisted document storage easier and also allow the server to skip the document parsing and validation stages for known/frequent documents.

[IvanGoncharov]

@williambailey Great suggestion 👍
I think the ability to replace parse is a great feature.
If you want to work on PR it should be very similar to #391

extend/augment/replace the new Source(...),

Source is a very primitive class so I don't think we should bother to replace it.
You can always access the original query as source.body.
https://github.com/graphql/graphql-js/blob/cd80f94edc584c5cf3258d27b6f91ab3b72f0cce/src/language/source.js#L31-L44

[IvanGoncharov]

I'm closing this issue since progress is tracked in #484