Default marker edge width in plot vs. scatter

[jhamrick]

The default marker edge width is inconsistent across Axes.plot and Axes.scatter, due to differences between lines and patches. When all the other defaults are updated, perhaps this could also be made consistent.

For plot, the edge width is currently 0.5 (from lines.markeredgewidth), and for scatter, it is 1.0 (from patch.linewidth).

[has2k1]

Maybe do this with new rcParam marker.linewidth.

[tacaswell]

Close this as the new defaults for these values are the same.