Support for Style (and/or Size) in ecdfplot

Over the past few years I've frequently find the need to plot cdfs of 2 or 3 orthogonal parameters and compare them. I've had to leave seaborn and use my own hacked cdf plots when this happened but it would be nice if it supported it directly.

you can see a thrown together implementation in https://github.com/mwaskom/seaborn/pull/3888

it supports the basic use case but has slightly uglier legends when both style and hue are used.