Automatic popover arrow placement

Created by: alecpl

Using the latest build based on Popper.js I don't see how to automatically place the popover arrow. When the button is on an edge of the screen the arrow is still in the middle of the popover.

NOW:-----------------------------------------------------------------------|
                                                                  ---------|
                                                                  |  link ||
                                                                  ---------|
                                           ---------------^----------------|
                                           |           popover            ||

EXPECTED:------------------------------------------------------------------|
                                                                  ---------|
                                                                  |  link ||
                                                                  ---------|
                                           ---------------------------^----|
                                           |           popover            ||

I see in the Popper demos it can position the arrow properly, so this is Bootstrap specific issue. Because the arrow uses :after selector it might be tricky to fix it programmatically. I suppose arrow positioning should be delegated to Popper.