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.