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.
```php
if ($this->getContainer()->has('liip_monitor.reporter.swift_mailer') && !$input->getOption('no-mailer')) {
```