Add color modifiers

[adamwathan]

This PR adds new color modifiers by default like prose-indigo, prose-blue, prose-pink, etc. that update link colors to match the modifier color. Useful for adding a little bit of branding to your content sections.

It uses the 600 shade of each color in your theme.colors configuration by default, excluding gray. If a color doesn't have a 600 shade it is skipped.

This is implemented simply as a default config, and not as any special logic inside the plugin. You can control which ones are enabled just like the size modifiers:

// tailwind.config.js
module.exports = {
  theme: {
    // ...
  },
  plugins: [
    require('@tailwindcss/typography')({
      modifiers: ['sm', 'lg', 'blue', 'pink'],
    }),
    // ...
  ],
}