PluginBundle and Hook support

Initial support for simple plugins.
The `HookSystem` allows listening to "hooks", which can be executed via PHP or through a `hook` Twig function.
This is checked at [ElcodiPluginExtension#L91](https://github.com/elcodi/elcodi/pull/409/files#diff-67cff955c6b90c027adee2e4f9e49126R91) but I can remove this.