Adding support for custom babel configuration

[tuchk4]

With this PR It is possible to override builtin babel presets and plugins via .babelrc or package.json.

Note: Don't forget to install "babel-preset-react-app" and include "react-app" preset into your .babelrc or package.json babel configuration section.

via .babelrc

{
  "presets": ["react-app", "stage-0"]
}

or via package.json

...
"babel": {
    "presets": ["react-app", "stage-0"]
  }
...

Now is possible to use:

• Decorators
• Features from stage-0 preset
• Any new babel features and presets

NOTE: If using custom babel with "babel-preset-react-app" preset - you will get warning message if "babel-preset-react-app" latest version is not range of your local version

[gaearon]

Thanks for the PR! We definitely won't be supporting Babel config in the close future, for reasons described in many past issues and PRs concerning this. Cheers!