PsrMessage DTO for extended context handling

[mrodikov]

PSRTarget is currently cast a logged message to a string, and there is no way to pass the context in any way since Yii does not intend to add the context to the message from outside.

The new DTO solves this problem by, on the one hand, adding the ability to transmit both message and additional context at the same time, and, on the other hand, leaving the ability to use itself for standard Yii log targets by implementing __toString method.

Usage example:

$psrLogger = new \Monolog\Logger('my_logger');
$psrLogger->pushHandler(new \Monolog\Handler\SlackHandler('slack_token', 'logs', null, true, null, \Monolog\Logger::DEBUG));
$psrLogger->pushProcessor(new \Monolog\Processor\PsrLogMessageProcessor()); // PSR-3 message processing

Now we can add additional context to our messages (and use PSR-3 placeholders populated by our context data):

Yii::error(new PsrMessage('Critical message', ['my' => 'context']));
Yii::getLogger()->log(new PsrMessage('Critical message from {process}', [
    'process' => 'BANANA',
    'other' => 'data',
]), Psr\Log\LogLevel::CRITICAL);

[samdark]

Looks good to me. Would you please add an example of using it to readme? Then it's good to merge.

[mrodikov]

Sure I will (tomorrow)

[mrodikov]

Done