Allow enabling/disabling modules and specifying variants from config file

[adamwathan]

Still WIP

This PR does a bunch of stuff:

• Makes module names more consistent so each generator matches the name we want to expose to the user for that module

• Adds a new modules key to the config file where you can customize how Tailwind generates each module

• Missing keys in the config now use the default values for those keys; this was already happening in some places but it's now consistent

• Allows specifying the generated variants for a given module in the config file, including not generating responsive variants for a given module if desired:

  {
      // ...
      options: {
          // ...
          modules: {
              // ...
              appearance: [],
              backgroundColors: ['responsive', 'hover'],
              zIndex: ['responsive'],
          }
      }
  }

• Allows disabling a module:

  {
      // ...
      options: {
          // ...
          modules: {
              // ...
              whitespace: false,
          }
      }
  }

• Moves hard-coded decisions about which utilities get which variants by default into the default config file instead

Still to do:

• Add support for focus variants in config file
• Refactor the generateUtilities plugin to make it easier to write some tests for the new configuration features
• Write those tests