Add `variations` option

This PR adds the ability to add asimov-style variation classes to the modal container as requested by the handover team. This can be done by either add a a space delimited string of variations as a `variations` option to the plugin, for via the dom `data-modal-variations` attribute.

```
<div class="modal__container" data-modal-variations="foo bar"></div>
```

Results in 
```
<div class="modal__container modal__container--foo modal__container--bar" data-modal-variations="foo bar"></div>
```