Feature Request: Allow composition of computed decorators

For feature parity with ember-macro-helpers and ember-awesome-macros.

This works:

```javascript
import EmberObject from 'ember-object';

import or from 'ember-awesome-macros/or';
import computed from 'ember-macro-helpers/computed';

export default EmberObject.extend({
foo: computed(or('bar', 'qux'), function(barOrQux) {
})
});
```

This, sadly, does not:

```javascript
import EmberObject from 'ember-object';

import computed, { or } from 'ember-computed-decorators';

export default EmberObject.extend({
@computed @or('bar', 'qux') // or @computed(or(..))
foo(barOrQux) {
}
});
```

This works (with kellyselden/ember-awesome-macros#348):

```javascript
import EmberObject from 'ember-object';

import mapBy from 'ember-awesome-macros/array/map-by';
import sum from 'ember-awesome-macros/sum';
import raw from 'ember-macro-helpers/raw';

export default EmberObject.extend({
items: [],

totalQuantity: sum(mapBy('items', raw('quantity')))
});
```

This, sadly, does not:

```javascript
import EmberObject from 'ember-object';

import { mapBy, sum } from 'ember-computed-decorators';

export default EmberObject.extend({
items: [],

@sum @mapBy('items', 'quantity') // or @sum(mapBy(..))
totalQuantity
});
```