Handle loading config fragments #1

It'd be nice to load config in multiple parts, like reading some parts from distribution defaults, while overriding some other parts in an own config, without copying all parts from distro defaults. This is complicated however, as a config fragment that doesn't contain all data to fully deserialize is not something that facet-toml supports. Needs careful consideration to figure out what to do with this. Maybe we can work with Partials from Facet, maybe we'll have to come up with something ourselves. What I don't want to be necessary is that the user maintains two variants of the struct, one with everything optional and one proper one.