Add support for error extensions

[blaedj]

This commit:

• adds an ErrorExtensions attr to the graphErr type
• adds an Extensions method to the graphErr type

Why?

• This adds support for error extensions as described in a
  recent(ish) update to the graphql spec.
  The interface changes are minimally invasive, and are in fact
  invisible unless the importing code defines an interface that
  includes the Extensions() method.

An example usage:

package foo

import "github.com/machinebox/graphql"

type extendedError interface {
	Extensions() map[string]interface{}
	Error() string
}

func bar() {
   client := graphql.NewClient(...)
   //...
   err := c.client.Run(ctx, req, resp)
   if ok := err.(extendedError); ok {
       extMsg, _ := json.Marshal(ee.Extensions())
       fmt.Println(extMsg)
      // prints out the extensions, e.g.:
      //{
      //  "message": "Name for character with ID 1002 could not be fetched.",
      //  "locations": [ { "line": 6, "column": 7 } ],
      //  "path": [ "hero", "heroFriends", 1, "name" ],
      //  "code": "CAN_NOT_FETCH_BY_ID",
      //  "timestamp": "Fri Feb 9 14:33:09 UTC 2018"
      //}
   }
}