Support configuration for different environments.

[yavorsky]

Currently, with .babelrc you can use the env option to set specific options when in a certain environment:

{
  "env": {
    "production": {
      "plugins": ["transform-react-constant-elements"]
    }
  },
  "presets": ["es2015"]
}

will use preset-es2015 for all envs and add transform-react-constant-elements just for prod.

I'll work on this.

[denysdovhan]

May cause conflicts with #54.

[yavorsky]

@denysdovhan why? we just need to support .babelrc (.babelrc.js for Babel 7)