0.17.0

Added

• Topology: undo and group moves (#55, from Sam / @slmingol). Ctrl+Z
  or Cmd+Z steps back up to ten node moves. Shift+drag on the background
  draws a lasso; dragging any selected node moves the group together,
  spacing kept, and one undo restores the whole group. Escape or a
  shift-click on the background clears the selection. Landed with two
  fixes: undo redraws the node even after the simulation has cooled, and
  it restores the node's saved state instead of turning a settled node's
  hold into a pin.