`generate`: Bundle a `config` template

[simonsan]

I think it would be beneficial to bundle a fully annotated config template in the compiled binary with:

pub static CONFIG_TEMPLATE_TOML: &str = include_str!("config/config_template.toml");

and let users generate a template config with rustic generate config > <path_to_future_config> or rustic generate config -o <path_to_future_config>, if -o/--output is given by the user.

Besides, I think it would be beneficial, as well, to package and ship rustic with the /config/ folder and maybe unpack it on installation to ~/.config/rustic/examples.