Add .col and .row-* classes to support flex-direction: column.

Created by: cgodo

I saw there is no support for flex-direction, not being set anywhere so it uses the default value of "row".

I suggest to add .col and .row-* classes to support flex-direction: column. Those classes would be the same as current .row and .col-* classes, except that .col class would use flex-direction: column.

So Bootstrap 4 could have (using xs and 12 for this example):

<div class="row"> <!-- uses flex-direction: row -->
  <div class="col-xs-12"></div>
</div>

<div class="col"> <!-- uses flex-direction: column -->
  <div class="row-xs-12"></div>
</div>

<div class="row"> <!-- nesting -->
  <div class="col-xs-6 col">
    <div class="row-xs-6"></div>
    <div class="row-xs-6"></div>
  </div>
  <div class="col-xs-6 col">
    <div class="row-xs-6"></div>
    <div class="row-xs-6"></div>
  </div>
</div>

Of course, there should be some study about what do "xs", "md" and "lg" mean in the context of "column" layout. Media queries with screen height I suppose?

Cheers.