expose `legendProp` in chart props

[tlowande]

[tried using pie chart, works] we could expose legendProp={} to the chart, that'd be used by getCommonProps and could override any legend details.

This is already a prop of getCommonProps but can't be passed from the chart apparently. So say if I want the legend to be square and not a circle, I could override it without changing the chart. That's a very small feature though but just adds to the flexibility of anything Legend related though