run pack with just an element map

Allow `pack` to run with just an element map definition. This will allow `pack` to run for n number of items.

```javascript
//my-elements.js
module.exports = {
  'el-one': '@foo/bar@1.0.0'
}
```

```shell
pie pack --elements ./my-elements.js
```

* How to handle element naming - say 2 element names pointing at the same dependency?