Add print variant

[reinink]

This PR adds a new print variant, which allows to add styles specifically targeted for print media. For example:

<nav class="print:hidden">
  <!-- Hide menu when printing -->
</nav>

<h1 class="text-blue-700 print:text-black">
  <!-- Make text black when printing -->
</h1>

This will generate the following CSS:

@media print {
  .print\:hidden {
    display: none;
  }
}

@media print {
  .print\:text-black {
    color: black;
  }
}