User-defined priorities can be added at runtime using the Log's
addPriority() method:
<?php
$logger->addPriority('FOO', 8);
The snippet above creates a new priority, FOO, whose
value is '8'. The new priority is then available for logging:
<?php
$logger->log('Foo message', 8);
$logger->foo('Foo Message');
New priorities cannot overwrite existing ones.