Support newer webpack version and plugin support

[dpd73]

Hi there,

Thanks for this plugin, it has been useful while I try and migrate a legacy app from grunt to webpack.

However #14 ought to be merged to support newer versions of webpack, otherwise you'll encounter:

[webpack-cli] TypeError: compiler.plugin is not a function
...SNIP...

for anyone else seeking to use this plugin, the fork has been published https://www.npmjs.com/package/assemble-webpack-new

Also, any idea on how to integrate assemble plugins? https://assemble.io/plugins/ In particular https://github.com/assemble/grunt-assemble-permalinks

Thanks again