add option to pass in buildSandboxGlobals function

[arenoir]

This PR fixes issue #76 by exposing a bildSandoxGlobals config option in prember.

//ember-cli-build.js

const EmberApp = require('ember-cli/lib/broccoli/ember-app');
const environment = EmberApp.env();
const fastBootConfig = require('./config/fastboot.js')

module.exports = function (defaults) {
  let app = new EmberApp(defaults, {
    prember: {
      buildSandboxGlobals: fastBootConfig(environment).buildSandboxGlobals
    }
  }
 return app.toTree();
}