Adds a plugin to be used as a CSS processor.

PostCSS plugin can be in 4 formats:

• A plugin in Plugin format.
• A plugin creator function with pluginCreator.postcss = true. PostCSS will call this function without argument to get plugin.
• A function. PostCSS will pass the function a @{link Root} as the first argument and current Result instance as the second.
• Another Processor instance. PostCSS will copy plugins from that instance into this one.

Plugins can also be added by passing them as arguments when creating a postcss instance (see [postcss(plugins)]).

Asynchronous plugins should return a Promise instance.

const processor = postcss()
  .use(autoprefixer)
  .use(postcssNested)