Added renderLegendBlock prop

Added the renderColorLegend render prop to allow customizing how the color legend elements are rendered. For example, useful to add tooltips (compare the renderBlock prop): Story.

Thanks @AliceLeyou for the contribution!

<ActivityCalendar
  data={data}
  renderColorLegend={(block, level) => (
    <MuiTooltip title={`Level: ${level}`}>{block}</MuiTooltip>
  )}
/>