Issue description

I want to create a new folder called reports, which should not be part of the output.path of the webpack bundle. It should be a separate path, that I can choose with an option, to generate the stats.json file there.

Technical info

• Webpack Bundle Analyzer version: 2.13.1
• Webpack version: 3.6.0
• Node.js version: 8.9.4
• npm/yarn version: 5.6.0
• OS: Windows 10 (64 Bit)

Debug info

How do you use this module? As CLI utility or as plugin?
As a plugin, with options

If CLI, what command was used? (e.g. webpack-bundle-analyzer -O path/to/stats.json)
webpack-bundle-analyzer ./dist/stats.json (Only for reading the generated file with a custom npm script.)

If plugin, what options were provided? (e.g. new BundleAnalyzerPlugin({ analyzerMode: 'disabled', generateStatsFile: true }))

{
   generateStatsFile: true,
   analyzerMode: 'static',
   openAnalyzer: false
}

What other Webpack plugins were used?
CopyWebpackPlugin
SpriteLoaderPlugin

It would be nice to also attach webpack stats file.
It can be generated using these options:

new BundleAnalyzerPlugin({
  analyzerMode: 'disabled',
  generateStatsFile: true,
  // Excludes module sources from stats file so there won't be any sensitive data
  statsOptions: { source: false }
})`

stats.json will be created in Webpack bundle output directory.