Re–use existing font-feature-settings declarations

[marek-saji]

Declaration with:

selector
{
   font-variant-numeric: tabular-nums;
   font-variant-caps: all-small-caps;
}

Produced two font-feature-settings, first one being overwritten by first one:

selector
{
  font-feature-settings: "tnum"; /* has no effect */
  font-variant-numeric: tabular-nums;
  font-feature-settings: "smcp", "c2sc";
  font-variant-caps: all-small-caps;
}

Changed so that if rule already has a font-feature-settings (defined in source CSS or created by postcss for some previous font-variant-*), it gets reused.

[MoOx]

Thanks for this awesome PR. I'll handle that really soon when checking if module need update for postcss 4.

[MoOx]

Thanks a ton. Released as 1.0.2.