Subtle seperator mixin

/* Examples: */

li + li {
  @include subtle-sep(top, 1px, rgba(black, 0.2));
  // simulates border-top: 1px solid rgba(black, 0.2); 
}

li + li {
  @include subtle-sep(left, 1px, red);
  // simulates border-left: 1px solid red; 
}