Feature to build with just webpack
Hey.

I don't want to pretend to understand what goes on in skpm-builder but I'm wondering whether it would be possible to just build the .sketchplugin by running `webpack` itself, rather than through skpm-builder. 

Right now it gives you the webpack.skpm.config.js to provide webpack overrides, but I would prefer having a skpm webpack config I can extend, and run `webpack` directly, instead of through a custom command.

The use-case here for me is mostly related to security, where the pipelines I work with will only allow building/bundling by the use of `webpack` command. So in my scenario, any webpack-compatible configuration, no matter how crazy, is completely fine, but a custom script is not :\
