Refactor to support plugin like babel

Babel plugin object example:

type PluginObject = {
  pre?: Function;
  post?: Function;
  manipulateOptions?: Function;

  visitor: ?{
    [key: string]: Function | {
      enter?: Function | Array<Function>;
      exit?: Function | Array<Function>;
    }
  };
};

each babel plugin returns an object like this.

we should think in CreateGraphQL PluginObject as well.

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• Separate mongoose schema to a plugin;
• Separate relay templates to a plugin (figure out how to best do this);
• Create boilerplate projects for Create-GraphQL plugins;
• Handle plugins property on .graphqlrc.