Support setting the MessageCodeResolver format

In traditional non-Spring Boot app I would set MessageCodeResolver format like:

    @Configuration
    @EnableWebMvc
    public class WebConfig extends WebMvcConfigurerAdapter {

        @Override
        public MessageCodesResolver getMessageCodesResolver() {
            DefaultMessageCodesResolver messageCodesResolver = new DefaultMessageCodesResolver();
            messageCodesResolver.setMessageCodeFormatter(DefaultMessageCodesResolver.Format.POSTFIX_ERROR_CODE);
            return messageCodesResolver;
        }

Would be nice if Spring Boot could support this automatically by setting the format via an application property like:

    spring.messages.resolver.format: prefix # prefix or postfix

See http://docs.spring.io/spring/docs/4.0.3.RELEASE/javadoc-api/org/springframework/validation/DefaultMessageCodesResolver.Format.html