feature request : completion outside of php files

It's not uncommon (for example in a Symfony project) to specify class names (FQN) in a xml or yaml file.

In those case, having a specific completion backend would be nice but I wonder how feasible this is.

* first, phpactor doen't yet provide a completion rpc call that would work in such case
* but if that was solved, I'm not really about how things should be :
** should it be a specific ``company-phpactor-fqn`` completion backend that could be added to other modes ?
** does the context of buffer matter ? ie should we activate such a completion backend only in a php project and if yes, by which means ?
** should the completion only focus on typing the FQN (that's the use case I have in mind, for symfony files like ``services.yaml``)