Defeatureify

**Do not merge this PR yet, it contains an example commit!**

Per recent conversations in #ember-dev, this adds support for feature conditionals as described [here](https://gist.github.com/thomasboyt/a5f4e04e83365eb40afd).

It relies on the `defeatureify` NPM package, so to build a non-Canary build, you'll need to install it:

```
npm install -g defeatureify
```

This PR includes a commit that adds two example "features" that just `console.log` some stuff when you create an application, as well as an example `features.json`. Once that's signed off on as how we want to proceed, I'll remove it + rebase so this can get merged in :)