[Enhancement] Adding moment computed fixes #6

This PR is dependent on #9.  Will squash once you thumbs up and merge #9 .
i wonder if this can be done without ambiguity.

I have thought in the past it might be nice to add something like: `Ember.l` or `Ember.literal`, so we could do.

```js
a: moment('prop', 'format'),
b: format('prop', literal('format')),
c: format('prop', l('format'))
```