feature request: border positive classes (adding specific borders instead of removing)

[yuvii]

I really like the .border class. However, I find I constantly add this to my code:

@border : 1px solid #e9ecef !important;
.border-top    { border-top:    @border; }
.border-right  { border-right:  @border; }
.border-bottom { border-bottom: @border; }
.border-left   { border-left:   @border; }

(I'm using Less, but you get the idea).

It's a quick addition which makes things a bit more readable, so instead of doing this:

class="border border-top-0 border-left-0 border-right-0"

I do this:

class="border-bottom"

I think it makes sense, and it's really easy to add.

[mdo]

This was added in #25021.