Allow overlapping for minimal window borders

[ArkadyBuryakov]

Right now with draw_minimal_borders yes we can draw borders between terminal space that looks very compact.
However it takes a little bit extra space and looks odd on with a thin border width.

It would be cool to take this option even further and allow borders to overlap, causing even thinner sleek-looking borders.
It could be either:

• new value for draw_minimal_borders = overlap
• new config option overlap_minimal_borders yes

Color for the border should be selected in following priority order:

1. Active window
2. Bell window
3. Inactive window

Here is a diagram that explains what i mean:

[kovidgoyal]

In principle, sure, but I dont have the time for this, PR is welcome.