Expose optional features

There is currently no public API for accessing the state of optional features at runtime, and this addon seems like the ideal place. Something like:

```javascript
import { hasOptionalFeature } from 'ember-compatibility-helpers';

if (hasOptionalFeature('jquery-integration')) {
// ...
}
```

I think this will either require, or else supercede https://github.com/emberjs/ember-optional-features/issues/10, but either way, should address https://github.com/emberjs/ember-optional-features/issues/10#issuecomment-421180959.