examples: adds simple example for concurrent-resolvers

[chris-ramon]

Overview

• Built on top of: executor: thunks return signature improvement #393
• examples/concurrent-resolvers: A simple example that shows how to resolve fields
  concurrently using thunks, with & without the use of Goroutines.
• closes: parallelize resolvers #106

Test plan

• Manual test:

(graphql)-> go run examples/concurrent-resolvers/main.go | jq
{
  "data": {
    "concurrentFieldBar": {
      "name": "Bar's name"
    },
    "concurrentFieldFoo": {
      "name": "Foo's name"
    }
  }
}

[coveralls]

Coverage remained the same at 91.629% when pulling bbe1508 on examples-concurrent-fields into cd6ef8d on master.