V2 -- Update copying static files to copy-webpack-plugin

[sthzg]

In #54 we've updated the build step to support copying a static directory to dist. With #49 we are working on solutions that support running and building on non-root server directories. This requires the static files directory to be copied over to a possibly deeper location than /static/.

To achieve this, we need access to Webpack's stats object, which we don't get easily via a build step in the copy run script. Refactoring the copy operation to copy-webpack-plugin will resolve that and integrate it nicely into the Webpack workflow.

@weblogixx do you have any objections to this? Otherwise I would go ahead integrating it.