Add support for `<style>` inside SVG mixin

[davidkpiano]

Use case: animated inline SVGs! (Still need to test if this will actually work)

@include svg {

  circle {
    @include svg-style((fill: orange, transition: all 0.3s ease-in));

    &:hover {
      @include svg-style((fill: black));
    }
  }

  @include svg(circle, (...));
}