Add device orientation columns

[MarcoDeJong]

When working with BS3, I encoutered that there is no device orientation support beneath the first breakpoint. E.g. On iPhone portrait .col-xs--6 fits great, when turning to landscape however, there is plenty space to use more columns, for example .col-xs--4.

Should we add a .col-xs-landscape-* scheme within (orientation:landscape) mediaqueries? Or is there some other option which I oversee?

[cvrebert]

Already under consideration for Bootstrap v4.