Add !font-bold-style important modifier

[adamwathan]

Resolves #92, resolves #34.

This PR adds support for making any utility "important" by throwing a ! at the beginning:

<p class="font-bold !font-medium">
  This will be medium even though bold comes later in the CSS.
</p>

This matches the syntax we have been showing as an example of custom variants in the official documentation for the last 500 years:

As well as the syntax Netlify is using as they refactor their codebase to Tailwind:

There are probably weird things you can do with your CSS that will make this break, but that's why we release it and let everyone do their worst and see what is reported 😄

In the future I could see a world where instead of making things !important we use the :is selector and a private ID to boost the specificity without introducing conflicts with inline styles:

:is(#__tw, .\!font-bold) {
  font-weight: 700;
}

Browser support is pretty good for this but might want to wait until Safari 12/13 are a little more dead.