Missing phonenumber option in link plugin 

Link plugin of cascade plugins has [phonenumber option if project has phonenumber_field package](https://github.com/jrief/djangocms-cascade/blob/2.1/cmsplugin_cascade/link/forms.py#L91)
But in Django Shop plugin it is missing due to overriding the ```LinkForm.LINK_TYPE_CHOICES``` property..