Add a `standard` option

[pascalduez]

#dummy.yml
foo: 'value'

.sowieso {
  content: map(dummy, foo);
}

Would be transpiled to:

:root {
  --dummy-foo: 'value';
}
.sowieso {
  content: var(--dummy-foo);
}

[TrySound]

@pascalduez Seems like extra functionality. One plugin should do one thing. And in this case better will be just use yaml map and just copy it to :root.

[pascalduez]

Agree, the plugin would just output the :root declaration. One would need to pipe the custom-properties plugin after the map one. Still need some thinking though.

[pascalduez]

This is already covered by postcss-custom-properties now.