Added sub-generator setup-env #278

[sthzg]

This subgen creates a new Webpack config environment by

• creating conf/webpack/.js
• creating src/config/.js
• requiring and exporting the new env in conf/webpack/index.js

The commit introduces a basic config template that is supposed to be populated by the generator's users. Various more fine grained subgen options can be added at a later time (e.g. prompting the user if new run scripts should be added to package.json).

The subgen's basic functionality is backed up by unit tests that check

• if files are created
• if conf/webpack/index.js contains correct import/export

[coveralls]

Coverage decreased (-0.2%) to 96.016% when pulling 7a7481335a4e9c8112e397a0f3e0144874b91183 on feature/gh_issue_278 into a2bd75a83b7379bdc61c4fdc340dada9790015c7 on master.