Webpack 4 support

[attilapuskas]

We have noticed that with webpack 4 there is a DeprecationWarning for the plugin.

DeprecationWarning: Tapable.plugin is deprecated. Use new API on ``.hooks`` instead`
`at WebpackBundleSizeAnalyzerPlugin.apply (.\node_modules\webpack-bundle-size-analyzer\build\src\plugin.js:15:18)

According to that comment the changes should be done here.

[robertknight]

Thank-you for the report and the discussion link. I'm not actively using webpack for work at the moment, so I can't promise when I'll get to this. I will take a look at any PRs that are submitted for it.

[spiltcoffee]

@robertknight pretty easy fix, just changed stats.plugin('done') to stats.hooks.done.tap()

Confirmed it worked on my own Webpack 4 project.

[spiltcoffee]

@robertknight any word on #48 being merged?