Add postcss-apply plugin

[pascalduez]

Hi,

I'm not 100% sure about the plugin order, since it's supposed to be really similar with the custom properties one, I placed it next.
So basically all other transformations will happen on the applied rules, instead of the stored rules. Maybe in case of huge sets usage it could have an impact?

I added tests locally to the plugin to ensure it doesn't interfere with the custom properties one.

Refs #203

[kevinSuttle]

Looking forward to this landing.

[zeke]

So exciting!

[ben-eb]

&?

[pascalduez]

Replicated from above, but probably better that's right.

[MoOx]

Will this works with custom props referenced in the spec like this?

:root {
  --toolbar-theme: {
    background-color: hsl(120, 70%, 95%);
    border-radius: 4px;
    border: 1px solid var(--theme-color late); /* IS THIS SUPPORTED */ ?
  };
  --toolbar-title-theme: {
    color: green;
  };
}

.toolbar {
  @apply --toolbar-theme;
}
.toolbar > .title {
  @apply --toolbar-title-theme;
}

I guess it don't since apply transformation is applied after custom props. But maybe I am missing something.

[pascalduez]

Will this works with custom props referenced in the spec like this?

I added some tests to the plugin for this very use case, seems okay to me: https://github.com/pascalduez/postcss-apply/blob/master/test/integration.test.js#L136-L227

Custom properties (single) are correctly parsed and replaced inside the custom properties (set), provided the plugins are in this PR order.

[MoOx]

Looks good enough. Thanks for this feature.

[MoOx]

Released as 2.7.0.