Allow setting multiple colors for different edges of border

[jk0893]

I wanted to simulate the windows 98 look with white color on left&top edges and black color on right&top edges, but apparently you cannot do such thing with even gradient.

It would be nice if we could set multiple colors with css-like gradient or, even better, for edge cases (pun unintended) specify edges like this:
col.active_border = <col1> <col2> (horizontal-vertical)
or
col.active_border = <col1> <col2> <col3> <col4> (left-top-bottom-right)
(so basically like margin/padding works in css).

I know there is already similar pattern but for gradient. For that user could provide gradient(). Idk, I'd just be happy to have such feature