Allow to add custom processors by service id

Followup from my previous pull request; https://github.com/desarrolla2/RSSClient/pull/19.

The processor concept is good, and it's working, except it was not possible in the bundle yet, so I added it.

Example:

```
rss_client:
    cache:
        adapter: Desarrolla2\Cache\Adapter\File
    channels:
       weblog: ["http://weblog.hostnet.nl/feed/"]
    processors: ["my_service_id", "my_other_service_id"]
```

This will add the custom processors to both the normal and the cached version. By default this is an empty array, resulting in no additional calls / overhead.

If you like this way we can take a look at the docs / update them accordingly.