Add YAML support

Since CiviCRM now allows registering of services in the container using the hook_civicrm_container, I would like to allow loading of a YAML service definition file.

I don't think this is the arena to debate which is the best way of doing things, but as it stands the existing dependencies allow using XML, PHP or ini file loaders. Since YAML is the default Symfony method for service definitions, I think we should at least offer the possibility of using it, and then debate on which method to push for in a best practices section.

I can create a PR just to add the dependency, I'm not sure whether here or the PR is the place to discuss it, but I saw that most PRs have an issue number, so I thought I should create one.

Also not sure whether according to semantic versioning this is a minor or path version bump.