[5.3] Add custom email subject support to scheduled tasks

[stevenmaguire]

These changes add an optional emailSubject attribute to the Illuminate\Console\Scheduling\Event which, when set, will become the subject for the notification email that is sent. If this option is not configured, the existing default behavior will resume.

$schedule->command('foo')
         ->daily()
         ->sendOutputTo($filePath)
         ->emailSubject('My awesome custom subject')
         ->emailOutputTo('foo@example.com');

This PR also includes an additional unit test to verify these changes. Due to the use of the global file_get_contents function and some of the gnarly bits inside the mailing infrastructure, this test case is not as elegant as I would like. It does however exercise the functionality and test the actual email message attributes rather than mock intermediate methods.

[taylorotwell]

You can already customize the subject by setting a description.

[stevenmaguire]

@taylorotwell While it is true that by setting a description it alters the email subject, the framework still forces the subject to include "Scheduled Job Output"; which is a reasonable default if email subject is not explicitly defined.

https://github.com/laravel/framework/blob/5.3/src/Illuminate/Console/Scheduling/Event.php#L844

    /**
     * Get the e-mail subject line for output results.
     *
     * @return string
     */
    protected function getEmailSubject()
    {
        if ($this->description) {
            return 'Scheduled Job Output ('.$this->description.')';
        }
        return 'Scheduled Job Output';
    }

If a consumer of this framework wants to customize this subject line, especially to not include "Scheduled Job Output" (an english-only opinion), why not enable that? What is the basis for the objection?