Add computed properties

• Added ember-cli-ramda-extended/utils/computed file to house a few computed property macros that leverage ramda.

import {pluck} from 'ember-cli-ramda-extended/utils/computed'

export default Ember.Controller.extend({
  myArray: [{a: 1, b:2}, {a:10, b:20}],
  onlyA: pluck('myArray', 'a')  // -> [1, 10]
})