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

[GromNaN]

Thanks proposing this new feature. I think can be useful in rare cases the processors are dependent on each other.

It needs to be rebased on master. Features of PHP 7.1+ can be used.

[GromNaN]

The spaceship operator is designed for this use-case.

Suggested change

	return $left['tag']['priority'] - $right['tag']['priority'];
	return $left['tag']['priority'] <=> $right['tag']['priority'];

[sudo-plz]

Closed in favor of #455