added reporter to send mail when check fails

The idea of the MailReporter cames, when I created a little Symfony Command which will executes the checks. In this case it will be helpful to send a mail when a check fails. So you can easily create a cron job which triggers the command and inform you automatically.
I think you should use `->canBeEnabled()` (http://symfony.com/doc/current/components/config/definition.html#optional-sections)