GraphQL root level directive syntax

In the last [GraphQL working group meeting](https://github.com/graphql/graphql-wg/blob/master/notes/2018-02-01.md#present-graphql-import) we discussed to enable GraphQL directives on a document/root level. This would allow for the following syntax which avoids the common "why is it commented out" confusion around graphql-import:

```graphql
@import(defs: [Post, Comment] from: "./other-types.graphql")
@import(defs: Message from: "./msg.graphql")

type User {
id: ID!
}
```

## Open questions

- [ ] Final syntax: `@import`, `defs:`, `from:`

Start with the linked GraphQL working-group meeting note and the proposed examples in this issue. No files, tests, or implementation entry points are identified; the work is done when the root-level directive syntax and the `@import` argument names have been resolved and the project behavior reflects that decision.

Written by the indexing model from the issue text.