Improve Relay generation
I think we need a flag on .graphqlrc to enable some Relay generation code tweaks

for instance a Relay GraphQL Type could have a  `globalIdField()`

Changes required on Type.js.template

```jsx
import {
  globalIdField,
} from 'graphql-relay';
import {
  NodeInterface,
} from '../interface/NodeInterface';
                                     
fields: () => ({
    id: globalIdField('Company'),
    _id: {
      type: GraphQLString,
      resolve: obj => obj._id,
    },
...

interfaces: () => [NodeInterface],                                                                                 
```