Nth offset option for column proposal

[corbanbrook]

I find that it is sometimes useful to be able to offset the nth-child indexing when using cycle.

I propose the addition of an additional option, $nth-offset to the column mixin which would allow one to offset by a certain number. It would default to 0 which would have no effect.

Applied like this:

@if $cycle != 0 {
    &:#{$nth-selector}(n) {
      margin-#{_opposite-direction($side)}: $margin-r * 1%;
      float: $side;
      clear: none;
    }

    &:#{$nth-selector}(#{$cycle}n + #{$nth-offset}) {
      margin-#{_opposite-direction($side)}: $margin-last * 1%;
      float: _opposite-direction($side);
    }

    &:#{$nth-selector}(#{$cycle}n + #{1 + $nth-offset}) {
      clear: both;
    }
  } @else {
    &:last-child {
      margin-#{_opposite-direction($side)}: $margin-last * 1%;
    }
  }