Priority for extractors

| Q             | A
| ------------- | ---
| Bug fix?      | no
| New feature?  | yes
| BC breaks?    | no
| Deprecations? | no
| Tests pass?   | yes
| Fixed tickets | #107 
| License       | MIT
| Doc PR        | need to be written
i think wouters comment actually should go here. you can sort right before iterating. 
and $priority is an unlucky name, its not the priority value but a sub list of extractors.

in the chainrouter, this looks like this:
https://github.com/symfony-cmf/Routing/blob/master/ChainRouter.php#L124

so you could call $extractors $sortedExtractors and then $priority $extractors :-)