Add builder pattern for Partial

[synek317]

It would be useful when working with multiple "preloaded" sources, e.g.

#[derive(Config)]
struct Cfg {
    foo: usize,
    bar: String,
}

fn main() {
    let cfg = Cfg::builder().preloaded(Cfg::partial().foo(42)).load();
}

Perhaps makes more sense in a more complex example

fn do_something(cli_args: Args, global_config: GlobalConfig) {
    let cfg = Cfg::builder()
        .preloaded(Cfg::partial().foo(cli_args.foo).bar(cli_args.bar))
        .env()
        .preloaded(Cfg::partial().foo(global_config.my_app.foo).qux(global_config.global.qux))
        .load()?;
}