Add responsive spacing utility classes

Created by: maryleloisa

It would be nice to have responsive spacing utility classes to manage gutters easily.

like p-xs-a-2 where xs is breakpoint and 2 is a kind of multiplation factor for $spacer value.

It would be also nice to keep abbreviations like md, lg for only breakpoints to reduce the confusion.

And classes like .m-xs-auto, .m-xl-auto would be great for responsively centering block elements.