Allow attributes to be altered through event subscribers

[j4-m]

We need a little more flexibility on how attributes are generated and I'd assume this isn't unusual.

#95

[brooke-heaton]

I second this request. We use the 'name' module and are unable to properly map First and Last names since these are not simple text fields. The current documentation does not seem to allow us to alter or create custom attributes based on Drupal fields.