Add responsive media-mixins shortcut

[dkrnl]

usage example:

.media-xs({
  div {
    border:1px solid red;
  }
})

result:

@media (max-width: 767px) {
  div {
    border:1px solid red;
  }
}

[mdo]

Similar to #13014.

[cvrebert]

Yeah, duplicate basically.

[dkrnl]

sorry ): close...