Add priority field to processor tags

[sudo-plz]

So that the order in which the processors are used by monolog can be determined.

Example config:

    Foo\Bar\Log\Processors\FirstProcessor:
        tags:
            - { name: monolog.processor, channel: http }

    Foo\Bar\Log\Processors\SecondProcessor:
        tags:
            - { name: monolog.processor, channel: http, priority: -10 } # last

    Foo\Bar\Log\Processors\ThirdProcessor:
        tags:
            - { name: monolog.processor, channel: http, priority: -20 } # very last