Support configuring a single list of variants to apply to all relevant plugins

[adamwathan]

Closes #851.

This PR makes it possible for users to specify a global variants configuration that should be applied to all utilities by assigning an array of variants directly to the variants property in their tailwind.config.js file:

module.exports  = {
  variants: ['responsive', 'group-hover', 'hover', 'focus']
}

This is useful for Purgecss users who just want everything available and don't want to micro-optimize at the individual plugin level.

No breaking changes here technically but plugin authors would be encouraged to update their plugins to use the new variants function made available through the plugin API to look up the variants for their plugin instead of using the config function.

- function({ addUtilities, config }) {
+ function({ addUtilities, variants }) {
    addUtilities(
      {
        '.content-center': {
          'align-content': 'center',
        },
      },
-     config('variants.alignContent')
+     variants('alignContent')
    )
  }

This will ensure that your plugin respects the users global variants configuration if they choose to use this feature.

[reinink]

Thank you Adam.

Long live modules: 'all'!!!!!!!!!!!!

Sincerely,
Jonathan