[FEATURE] allow to setup options of type function

Some options are handlers and must be defined as functions, this feature enables that.

Example:
```javascript
// you can define custom flags
customLaunchers: {
  'PhantomJS_custom': {
    base: 'PhantomJS',
    options: {
      // ...
      onResourceRequested: function (req, controller) {
        if (req.url.indexOf('http://localhost') !== 0) {
          controller.abort();
          return;
        }
      }
    }
  }
}
```