Serialize functions to be able to setup options of type function
feat: support functions in the config

Example:
```js
customLaunchers: {
  'PhantomJS_custom': {
    base: 'PhantomJS',
    options: {
      // ...
      onResourceRequested: function (req, controller) {
        if (req.url.indexOf('http://localhost') !== 0) {
          controller.abort();
          return;
        }
      }
    }
  }
}
```