FormMapper Relationship combine fields on display

[crofton77]

I would like to display the first and last name of the users in the related field.

Currently I can only pass a string to display a single field

$mapper->belongsTo('owner')->display('first_name');

A example of a way to do this would be to pass an array of fields and a string to join the fields

$mapper->belongsTo('owner')->display(array('first_name','last_name'),' ');