Include `extensions` field in GraphQLError.

The [GraphQL docs](http://graphql.github.io/graphql-js/error/#errors) specify a field called `extensions` is available for a GraphQLError. GraphQL Python does seem to have a ['slot'](https://github.com/graphql-python/graphql-core/blob/master/graphql/error/base.py#L13) for this.

Users could add the field using a wrapper, but it looks like something that could easily be added to the class – unless I'm missing something!