Added support for custom init-param in VaadinConfig

With this modification one will be able to set one or more custom init-param directly in VaadinConfig like the following:

vaadin {
..............
  initParams = [
    "param-name-1":"param-value-1"
  ]
..............
}

This is useful for example if you are using vaadin-push with spring-security and you have to set an init-value for the init-param AtmosphereInterceptor.