Add support for Multi-compiler mode

[PMCorbett]

I've ran into an issue using this plugin with multi compiler mode only outputting the last assets into the manifest. This is a problem that has been solved on the webpack-manifest-plugin, where they allow you to specify a cache object, which then adds to the manifest in every multi-compile.

I've had a go at getting copying their method into this plugin, which seems to work

var _ = require('lodash')
/**
 * Create a new StatsPlugin that causes webpack to generate a stats file as
 * part of the emitted assets.
 * @constructor
 * @param {String} output Path to output file.
 * @param {Object} options Options passed to the stats' `.toJson()`.
 */
function StatsPlugin (output, options, cache) {
  this.output = output
  this.options = options
  this.cache = cache || {}
}

StatsPlugin.prototype.apply = function apply (compiler) {
  var output = this.output
  var options = this.options
  var cache = this.cache

  compiler.plugin('emit', function onEmit (compilation, done) {
    var stats = compilation.getStats().toJson(options)
    cache = _.merge(cache, stats)
    var json = JSON.stringify(cache, null, 2)

    compilation.assets[output] = {
      size: function getSize () {
        return json && json.length || 0
      },
      source: function getSource () {
        return json
      }
    }

    done()
  })
}

module.exports = StatsPlugin

Is this something you'd be interested in incorporating into this plugin? And could I raise a PR to do so?

To give some context as why I want to do this; I am creating several themed bundles using themed-sass-loader within a webpack-rails app. Webpack-rails uses the manifest to load in the correct assets, and checks it for errors, etc., so an entry for each theme is needed.

[unindented]

Yes please! Open a PR with your changes, and a test if you can, and I'll merge it.

Thank you!

[rhys-vdw]

Just for another use case here, I'm building a service worker and the application script using multi-compiler mode (also with webpack-rails).

[unindented]

If anybody opens a PR with the necessary changes and tests, I'll merge it!