feat: Implement menu auto flipping

[FezVrasta]

This PR aims to implement #1602 and #284.
Supersedes #1232, and is a foundation for #810, #697, #637.

Basically, it adds react-popper to the equation, making it possible for the dropdowns to flip automatically in case there's not enough space on the bottom.

react-popper is based on Popper.js, which is the same library being integrated in Bootstrap v4 to replace Tether.

Right now there's a bug with single options dropdown, that, when filled with an option, are misplaced.
I'm investigating the issue but I wanted to gather feedbacks to see if it is worth it or you don't plan to merge this.

Thank you!

[agirton]

Hi @FezVrasta thank you for the PR and great work on Popper! I'm a bit concerned that it basically takes over the package. However, I know this feature is important to many of our users. This would be good to have after a 1.0 release.

[alexmattson]

What is the status on this?

[juanpujol]

Is there any plans to merge this?

[JedWatson]

Hey @FezVrasta thanks for the PR - I've used popper before and it's a great solution for this.

However, I don't want to build it directly into react-select, as it's not always necessary and I'm trying to slim the core package down... it should be possible to do this using a custom menuRenderer component, or if not let's focus on making our API more extensible so you could.

Would be great to get an example added (if it is possible), in the same way @bvaughn has done with react-select-virtialised to demonstrate how to use it.