Preserve HTML elements by default when purging unused styles

[adamwathan]

This PR introduces a new preserveHtmlElements option to the purge configuration that allows you to safelist all plain HTML elements, like p, blockquote, body, video, etc.

// tailwind.config.js
module.exports = {
  purge: {
    content: [
      // Paths...
    ],
    preserveHtmlElements: true,
  },
}

This helps avoid accidentally purging things like heading elements when your source files are in a format that compiles to HTML, like markdown (since your markdown won't actually contain the string h1 anywhere).

This option is set to true by default.

On its own this PR won't change anything for anyone really, but I plan to change things to purge all of base, components, and utilities going forward (not just utilities), as this was the only real footgun until now, and with the new typography plugin gaining traction it would be nice to make those styles purgeable by default.

[pikeas]

Does this make https://github.com/tailwindlabs/blog.tailwindcss.com/blob/master/tailwind.config.js#L12-L22 unnecessary for MDX files?

[adamwathan]

@pikeas Yep not necessary anymore, although what we've done there is even more optimized (but way past the point of diminishing returns).