Adjustable Outline Color for Custom Node Sockets

All blender's built-in node sockets have a black outline, while NodeSocketVirtual and any custom defined sockets have a grey outline. It would be nice to be able to change the outline color of custom node sockets, either to convey some other visual information, or to match the theme of blender's built-in socket types.

This could be implemented by adding a new function to bpy.types.NodeSocket: draw_outline(self)

This function would return a tuple of three or four (four if alpha is included) elements.

If custom node sockets are given this functionality, it will mean that they can be matched with the theme of blender's built-in node sockets, which would be desirable to be able to do in some situations. It also means that the outline color can be used to convey information as well as the fill color.