Escape commas in class names to workaround minifier bug

[bradlc]

Fixes #45

There seems to be a bug in cssnano (seen in the playground here) which splits selectors by commas, even when they are escaped, for example:

/* Input */
.grid-cols-\[1fr\,128px\] {
  grid-template-columns: 1fr 128px;
}

/* Output */
128px\],.grid-cols-\[1fr\{grid-template-columns:1fr 128px}

This PR works around this by adding two new util functions which extend the ones found in tailwindcss. These functions replace commas with \2c to avoid the issue.