[5.7] Make the channel for log mail driver configurable

[sebdesign]

Allows users to configure the channel for logging the emails when using the log mail driver.

By default the mails are logged to the default channel in config/logging.php, but users can configure a custom channel in the channels array. That would be handy because mail logs are often huge and messy, and it's not easy to debug along with stack traces and other log messages.

For example in config/logging.php

'channels' => [
    // ...

    'mail' => [
        'driver' => 'daily',
        'path' => storage_path('logs/mail.log'),
        'level' => 'debug',
        'days' => 14,
    ],
],

To specify the mailer to use a specific log channel instead of the default, users can set the log_channel key in config/mail.php.

'log_channel' => 'mail',

[sebdesign]

@taylorotwell thank you for merging! Would you be interested in having this feature in the documentation? Or having a log_channel => null as a default in the config/mail.php that ships with laravel/laravel? I can submit PRs for both repositories is you want.

[driesvints]

@sebdesign you can send in a PR to laravel/laravel

[sebdesign]

@driesvints done! laravel/laravel#4855