adds link class dropdown for tinymce alchemy_link dialog

This brings over the element class functionality that the default tinymce link plugin has.
Let me know if tests are needed and where to put them, I couldn't find any specs regarding the fields inside the link dialog.

To use, add the class list config inside the tinymce initializer.
```
Alchemy::Tinymce.init = {
    link_class_list: [
        {title: 'Blue-Green Link', value: 'blue-green-link'},
        {title: 'Primary Button', value: 'primary-btn'},
        {title: 'Large Primary Button', value: 'primary-btn large'},
        {title: 'Secondary Button', value: 'secondary-btn'}
    ]
}
```
The dropdown is hidden if no class list is defined.