Add a PlainTextHandler for command line and LoggerInterface (Monolog).

Add a PlainTextHandler for command line error handling and Psr\Log\LoggerInterface output.

Usage:

```
$logger = new Monolog\Logger('logger');
$whoops = new Whoops\Run;
$whoops->pushHandler(new Whoops\Handler\PlainTextHandler($logger));
$whoops->register();
```