Implementation of `Scrollspy`

[markusjwetzel]

<Scrollspy.Provider>
  <Nav>
    <Nav.Link toggle={Scrollspy} target="anchor-1">
      Anchor #1
    </Nav.Link>
    <Nav.Link toggle={Scrollspy} target="anchor-2">
      Anchor #2
    </Nav.Link>
  </Nav>
  <View>
    <Scrollspy.Anchor id="anchor-1" />
    <Text>Lorem ipsum...</Text>
    <Scrollspy.Anchor id="anchor-2">
      <Heading>Anchor #2</Heading>
    </Scrollspy.Anchor>
    <Text>Lorem ipsum...</Text>
  </View>
</Scrollspy.Provider>

All anchor refs can be stored by id on mounting. On Nav.Link click we need to scroll to the position of the anchor ref. Also we need to track the current active anchor and add active class to the active Nav.Link accordingly.