Add a prefix option

It would be great if there was a prefix option for the CSS that's being embedded. I wanted to use this form but a lot of the CSS is getting overridden.

For example:

```
new Card({
form: 'form',
container: '.container',
prefix: 'html#ecwid_html body#ecwid_body .ecwid div'
})
```

This would then make it so that all of the css classes added to the page would be prefixed with `html#ecwid_html body#ecwid_body .ecwid div`