Is there an app or option that allows phone to ring for emergency contacts only when silent or Do not disturb? I work sometimes till 4 am and my friends and the rest of the world obviously don't know in advance that I don't want to be called at 8 am.

Is there an app or setting that can allow me to have an emergency override list of people I need to be woken up for if a possible emergency arises but ignore all other calls until I wake up?  

I'm familiar with the Do Not Disturb option, but it has limitations that it blocks all calls. What if there is an emergency at night?

What if I want to allow certain callers during the Do not Disturb time? Is there a way to have an emergency list that allows calls during silent?
In Do Not Disturb there's an option to allow calls from Favorites and/or a specific group - "Allow Calls From".