add trace `legendItem` styles

I'm thinking of adding a new trace attribute container: legenditem

{
  x: [/* */],
  y: [/* */],
  legenditem: {
    textfont: {
       color: 'red'
       size: 12
     },
     marker: {
       size: 20,
       symbol: 'circle',
       line: {width: 2, color: '#444'}
     }
  }
}

to the scatter*, bar* and pie traces to make their legend item styling configurable. This would be an easy (easier than @alexcjohnson 's legend 2.0 idea 😏 ) solution to https://github.com/plotly/plotly.js/issues/2967, https://github.com/plotly/plotly.js/issues/2080, https://github.com/plotly/plotly.js/issues/1701 and https://github.com/plotly/plotly.js/issues/1560

By the way, I'm thinking a naming this new container legenditem and not legend as I'm thinking legend could eventually be used when we implement multiple legends where e.g. legend: 'legend2' would mean "put this trace's legend item in the second legend".

One more note on @alexcjohnson 's legend 2.0 idea: having legenditem in traces wouldn't make it obsolete. We could still add a way to turn annotations into legend items later.

Thoughts?

首先查看链接的四个 issue（#2967、#2080、#1701 和 #1560），以了解所请求的图例行为和未解决的设计问题。为 scatter*、bar* 和 pie traces 定义 trace 级 legenditem 容器的范围；完成的标准是已就样式要求和实现边界达成一致。