add range charts to chart types

[grain209]

Chart type: area-range

here's an example of how it should look
https://jsfiddle.net/adamkasprowicz/xm9jk5gh/13/

A simple config to render that chart:

const chart = new KeenDataviz({
  container: '#chart',
  type: 'area-range',
  results: [res1max, res2min]
});

Config props that this chart will always add/override:

{
  type: 'area',
  legend: false,
  labels: ['Max', 'Min'],
  stacking: 'normal',
  colors: ['#ff0000','#ff0000'] // use the same color for both
}

.c3-areas-Min {
  display: none;
}

Add this functionality to step and spline. New chart types:
area-range-step
area-range-spline

Add this functionality to bar. New chart type:
bar-range
example:
https://jsfiddle.net/adamkasprowicz/xm9jk5gh/17/

Add this functionality to horizontal-bar. New chart type:
horizontal-bar-range
example:
https://jsfiddle.net/adamkasprowicz/xm9jk5gh/19/