[Feature] need add support for bar itemStyle borderRadius

[yuicer]

What problem does this feature solve?

series.bar.borderRadius is hard to custom defined when using dataset, wish to support callback function so we can use just like itemStyle.color. or add more options for visualMap component

What does the proposed API look like?

series.bar.itemStyle.borderRaduis = (params)=> {if (dataHandle) return [4,4,0,0]; return [0,0,4,4]}