Add support for @global

[jantimon]

There's a great proposal by @devongovett in css-modules/css-modules#390 to introduce an @global at-rule.

I believe it makes the code easier to read than :global and also helps in cases like #75 where :global lead to parsing errors.

@global {
  @keyframes fade {
    /* ... */
  }
}

Compared to the current approach:

@keyframes :global(fade) {
  /* ... */
}

What do you think about implementing this (in a backwards compatible way)?

[jaggli]

Oh that's so much more intuitive. I would really appreciate that.