Add individual className options for replacer and container

This lets you do things like get (basic) Bootstrap stylings for free
using:

```
{
  replacerClassName: 'btn btn-default',
  containerClassName: 'dropdown-menu'
}
```

Though you'll need to add a little override to make that particular mix
work:

```
.sp-container.dropdown-menu {
  display: block;
}
```