Create custom directives

Is there a way to create custom directives? I was looking to implement authentication using directives by following this [blog post](https://blog.graph.cool/graphql-directive-permissions-authorization-made-easy-54c076b5368e), but could not figure out how to define a custom directive from the source code or godocs.

Here is an example from the blog post.
```
type Vehicle {
id: ID!
year: Int!
make: String!
model: Int!
askingPrice: Float
costBasis: Float @hasRole(role: “MANAGER”)
numberOfOffers: Int @isAuthenticated
}
```

It would be nice to be able to define an `isAuthenticated` directive that will determine whether or not a field should resolve.

Is there currently a way to implement this?