ENH Add colormap= keyword to DataFrame plots

Refactor color and colormap option handling

Add tests for colormap=

Add tests for colormap=

Add colormap documentation to visualization.rst

Add release notes
Merge pull request #3860 from qwhelan/plotting_colormap

Add colormap= argument to DataFrame plotting methods
BUG: Possibly invalidate the item_cache when numpy implicty converts a view to a copy (GH3970)

PERF: testing perf

BUG: implement cache tracking with a weak reference