BREAKING CHANGE: VictoryTooltip should not have a default theme

[boygirl]

fixes FormidableLabs/victory#719

this will be a breaking change for people who are using their own themes, but not defining any tooltip theme. To compensate for this change, add the following to your theme object:

tooltip: {
    style: assign({}, centeredLabelStyles, { padding: 5, pointerEvents: "none" }),
    flyoutStyle: {
      stroke: charcoal,
      strokeWidth: 1,
      fill: "#f0f0f0",
      pointerEvents: "none"
    },
    cornerRadius: 5,
    pointerLength: 10
  }