RenderProps for FocusableSection (maps to `leaveFor`) + 1 lifecycle update for removing sections

[chwagssd]

• <new RenderProp> created sectionId configuration to allow sections to utilize leaveFor option
• <new RenderProp> added 4 neighbor options for FocusableSection to allow for declarative navigation between sections
  • documented new properties
  • neighborUp/neighborRight/neighborDown/neighborLeft
    • mapped into the js-spatial-navigation config option for a section: leaveFor{up, right, down, left}
    • Allowed for empty string "", to force a wall at the edge of a section
• moved JsSpatialNavigation.add() inside of componentDidMount
  • Note: componentWillMount is being deprecated
  • Order is important, and when mounting new components, componentWillUnmount is called after componentWillMount, which means duplicates between routes can occur once routing is introduced
• removed package-lock.json because it was tied to free-bsd, which means cannot build on mac/windows, so let build env determine