Use dumper as service with new functs

Hi Vincent,

I've use this great bundle in a project (thanks) and have some ideas for it, so I implement it :)

First: Dump of feed file is now a service, it's used for the cmd dump and it could be used where ever there's a ServiceContainer

I have love the possibility to generate feed from none entity model, it work very well, but I couldn't dump it with the console cmd: It need to have an entity with your "export" interface... 
I've set the entity as optionnal, and if you don't have set it, you could hydrate you feed directly with your own model :D 

I've fix also a small issue that I get on my test server: dump feed with the cmd generate local's url... So, I look up how sonata sitemapBundle solve the problem and add a host arguments

This is my first "addon" on existing repo commit (not a typo fix), please be kind :rabbit: 
Can you add a space after `if (null !== $this->entity) {` (and maybe it could simply be `if ($this->entity) {`), what do you think?