make react-popper a peerDependency

[rheydt]

react-popper is a popular package that many react-overlays users might already be using. By specifying it as a dependency instead of a peerDependency, your users may be forced to install multiple versions/copies of react-popper.

Please consider making this a peerDep instead of a regular dependency. Thanks!

[taion]

It shouldn't cause issues unless you have incompatible dependency ranges. You can always just regenerate the lockfile if you have dupe versions.

[jquense]

likely that we will drop react-popper in the near future to use popper directly. react-popper is a great library, but for the limited use-cases here and thanks to hooks, it's probably lighter weight to use Popper directly now!