[RFR] Add callback field type

Usage:
```js
.addField(Field('actions')
                .type('callback')
                .list(true)
                .label('Actions')
                .isEditLink(false)
                .callback(function(entity) {
                    return '<a href="#/edit/posts/' + entity.post_id + '">View post</a>';
                })
            )
```
angular directives won't be interpreted if you use `ng-bind-html`, will they?