Allow altering field mappings after sync

In my project, couple staff members (real Drupal users) manage special entity type called contact, which represent their clients. All contacts have email fields and my task was to create ability to subscribe these contacts to newsletters. I tied custom subscription form with EMF and everything works quite good except one thing.

When new list is synced in, EMF assumes wrong default regarding 'mail' field. It always equals either $account->mail or [user:mail]. However, it's not desired thing in our case, where we'd want it to be [contact:email]. And since there's no way to alter default field mapping, it's not possible now. I've prepared a patch which allows you to change this using hook_emf_field_mapping_alter().