marker edge default color

In 0.61.0, the marker edge color is still black, instead of being the same as
the marker face color. Personally, I think they should be the same. What do
people think?

One reason for the same color argument is that if the markersize is set to a
small value, it will show mostly the edge color.

One possible reason against it is that if the marker color is white (or whatever
the background color is), then you can't see the marker. But we can default
this case to black (or whatever).

JC Hsu