Modularized loader syntax

This release features a new modularized loader syntax which allows users to load config values via the following new syntax, instead of first-level methods on the configSet struct:

configSet := confiq.New()

if err := configSet.Load(
    confiqjson.Load().FromFile("./config.json"),
    confiq.WithPrefix("somePrefix"),
); err != nil {
    log.Fatal(err)
}