Make promise-based titles work with FastBoot
Make promise-based titles work with FastBoot

Following up from #55, this makes FastBoot wait for our async code to complete before rendering the page.

The code checks that FastBoot exists and that we’re running on FastBoot before using [fastboot.deferRendering](https://ember-fastboot.com/docs/user-guide#deferring-response-rendering) to tell FastBoot about our Promise.

kimroen/ember-cli-document-title,kimroen/ember-cli-document-title