Hook for display name

Today I was working on customatisation of Contact_BAO_Contact show that it was possible to inject a custom display name generation algorithm. I could achieve this to put my custom code into the my overwritten Contact_BAO_Contact but I have added a hook into this function to so that it was possible to achieve this in a hook in my extension and so that I have only to add the hook invoke call to the BAO (less changes into the Contact_BAO_Contact class

While I was doing this I thought this could be an extra hook for the core. So I thought I will report it as an issue and add my code with pull request on github so that the fix is provided and the core team can decide if they want this behaviour in core or not.