How to enable the "Allow Peeking" option on Lollipop?
"Heads-Up" Notification is introduced in Android Lollipop. However you can only disable the feature by updating to Marshmallow. Marshmallow has the option for disabling Heads-Up notification so-called "Allow Peeking".

Below is the screenshots for Lollipop and Marshmallow.



App notifications on Lollipop



App notifications on Marshmallow