Add hook_inline_entity_form_settings_alter()

Today I was trying to write IEF integration for field collection fields. Field collection field is a reference type field similar to entity reference and I prefer the inline entity form widget to the one that comes with field collections. However inline_entity_form_settings() will return a controller only for a hard-coded list of field types.

Seems like there should be a "hook_inline_entity_form_settings" or something so that other modules can define field types that will work with IEF. I could provide a patch if you think this is a good idea.