Add scroll behaviour for container elements

[Mesoptier]

Dual of this PR on scroll-behavior

You can use the ScrollContainer component to wrap a scrollable element. The class will handle calling the new registerContainer and unregisterContainer methods on scrollBehavior. Under the hood the scrollBehavior instance is added to the React context, so this class can be used anywhere.

Example code:

import { ScrollContainer } from 'react-router-scroll';

function SomeComponent() {
  return (
    <ScrollContainer scrollKey="someComponent">
      <div style={{ height: 200, overflow: 'auto' }}
        <div style={{ height: 500 }}></div>
      </div>
    </ScrollContainer>
  );
}

[taion]

This is great. Let's get taion/scroll-behavior#86 merged and released, then I'll look at this in more detail.

[taion]

I'll clean this up. Thanks!