[5.7] Allow predefined log channels to change formatter from config

[troccoli]

This PR is about the ability to specify a formatter from the config file for the single, daily and syslog driver.

The default formatter, the LineFormatter, print the date and time, channel name, level and text for every message logged. To change the format of the message, for example to remove the channel name, or to use a different formatter, for example the HtmlFormatter, you have to use the monolog driver with the correct handler.

Compare the following configurations:

return [
    "channels" => [
        "formatted_single" => [
            "driver" => "monolog",
            "handler" => StreamHandler::class
            "handler_with" => [
                "stream" => storage_path("logs/laravel.log"),
                "level" => "info",
            ],
            "formatter" => LineFormatter::class,
            "formatter_with" => [
                "format" => "[%datetime%] %level_name%: %message%",
            ],
        ]
    ]
]

and

return [
    "channels" => [
        "formatted_single" => [
            "driver" => "single",
            "path" => storage_path("logs/laravel.log"),
            "level" => "info",
            "formatter" => LineFormatter::class,
            "formatter_with" => [
                "format" => "[%datetime%] %level_name%: %message%",
            ],
        ]
    ]
]