add sparkline to chart types

[grain209]

https://en.wikipedia.org/wiki/Sparkline

it's a pure chart like
https://jsfiddle.net/adamkasprowicz/8p20tgmL/2/
but without the lines in the back

to use it, people will set a config's property:

const chart = new KeenDataviz({
  container: '#my-chart-div',
  type: 'area', // or any other chart type with x,y axis
  sparkline: true
});