Add new variant in enum Layer to let the users add a custom layer

[bnjjj]

Add that kind of variant in Layer.

// the custom variant
Custom(Box<dyn Fn() -> Result<serde_json::Value, twelf::Error>>)

which would let us use this layer like this

let fetch_from_api = || {
    let res= reqwest::blocking::get(« http://myapi.com »).unwrap();
    res
};
Conf::with_layers(&[Layer::Env(None), Layer::Custom(Box::new(&fetch_from_api)]).unwrap();

the main goal is to let users fetch part of the configuration from whatever they want and not only from provided and supporter layers