Allow to configure source maps generation for different environments

[artembatura]

For example, currently we can do:

{
  "zero-scripts": {
    "webpack-config": {
      "useSourceMap": true
    }
  }
}

Proposed:

{
  "zero-scripts": {
    "webpack-config": {
      "useSourceMap": {
        "development": true,
        "production": false
      }
    }
  }
}