[shape][demo] add <Stack />, add /streamgraph

[hshoff]

There are two ways of handling the stack series data + path

<Stack
  data={cityTemperature}
  keys={keys}
  offset="wiggle"
  curve={curveBasis}
  x={d => xScale(x(d.data))}
  y0={d => yScale(d[0])}
  y1={d => yScale(d[1])}
  fill={d => zScale(d.series.key)}
/>

or you can use a render prop for total control of what gets rendered

<Stack
  data={layers}
  keys={keys}
  offset="wiggle"
  x={(d, i) => xScale(i)}
  y0={d => yScale(d[0])}
  y1={d => yScale(d[1])}
  render={({ seriesData, path }) => {
    return seriesData.map((series, i) => {
      return (
        <g key={`series-${series.key}`}>
          <path
            d={path(series)}
            fill={zScale(series.key)}
          />
          <path
            d={path(series)}
            fill={`url(#${patternScale(series.key)})`}
          />
        </g>
      );
    });
  }}
/>