🔧 Change plugins to expect array of objects

[markbrouch]

This PR fixes the plugins signature to expect an array of objects, matching the Pug options API.

This allows user to pass arguments to the plugin:

...
{
  loader: 'pug-html-loader',
  options: {
    plugins: [
      myCustomPlugin(options),
      myOtherPlugin(),
      {  // or you could even inline a plugin here
        resolve(filename, source, options) {...}
      }
    ]
  }
},
...

You could also pass in pluggable override hooks in an object and pug-html-loader will wrap them into an array to pass to Pug as plugins:

...
{
  loader: 'pug-html-loader',
  options: {
    plugins: {
      resolve(filename, source, options) {...},
      lex() {...}
    }
  }
},
...

This also matches the webpack signature for plugins, providing additional familiarity.