Add support for overwrites in the config builder

[TroyKomodo]

Allow config overwrites in the builder.

ie.

let mut builder = ConfigBuilder::new();

builder.overwrite("abc.key", "some value");

builder.overwrite("abc.vec", vec![1, 2, 3]);

let config: AppConfig = builder.build()?;

[TroyKomodo]

cc @lennartkloock