Add methods hCenter(to edge:) and vCenter(to edge:)

• Add methods to pin hCenter and vCenter to any other view's edges (including the new hCenter and vCenter edges)
  • New methods:
    • func hCenter(to edge: HorizontalEdge) -> PinLayout
    • func vCenter(to edge: VerticalEdge) -> PinLayout
  • New UIView's edges:
    • UIView.edge.hCenter
    • UIView.edge.vCenter

View build details and download artifacts on buddybuild:
PinLayout (iOS, PinLayout)
PinLayoutTVOS (iOS, PinLayoutTVOS)