Add extesion

[xeontem]

I use this plugin on the project. And now I need to write my own plugin for nunjucks. So, I have to add it by using 'addExtension'(https://mozilla.github.io/nunjucks/api.html#addextension) method from my configured env. but 'nunjucks-webpack-plugin' do not provide the env. Any ideas to solve this issue?

[alexander-akait]

@xeontem i think we can add setupEnv(env) { /* You logic */ } function option to allow this, feel free to Send a PR