"Add DecodeHooks also to "unmarshaling" env variables"

[sagikazarmark]

Moved here from #772, commented by @stepanselyuk:

---

I don't know how to formulate a behavior, but an example:
when I use a config file (e.g. YAML) with a key:

blocked_countries:
    eu: [AT,BE,BG,CY,CZ,DE,DK,EE,ES,FI,FR,GB,GR,HU,HR,IE,IT,LT,LU,LV,MT,NL,PL,PT,RO,SE,SI,SK]
    xx: [XX]

it reads that as:

"blocked_countries": map[string]interface {}{
            "xx": []interface {}{
                "XX",
            },
            "eu": []interface {}{
                "AT",
                "BE",
                "BG",
                "CY",
                "CZ",
                "DE",
                "DK",
                "EE",
                "ES",
                "FI",
                "FR",
                "GB",
                "GR",
                "HU",
                "HR",
                "IE",
                "IT",
                "LT",
                "LU",
                "LV",
                "MT",
                "NL",
                "PL",
                "PT",
                "RO",
                "SE",
                "SI",
                "SK",
            },
        },

but I want to rewrite this value using env variable: BLOCKED_COUNTRIES, and by the idea I should specify a json there, like {"eu":["DE","DK"]}, but it won't work, the blocked_countries won't be presented in viper.AllSettings(). I can affect the value only partially by using e.g. export BLOCKED_COUNTRIES_EU=DE,DK.

There is an option (#641) to specify DecodeHook, but only for unmarshaling. But on the step when I want to unmarshal what viper has into a struct, viper already does not have blocked_countries key using AllSettings(), though I can access a raw value of it using viper.Get("blocked_countries").

So my request is to add DecodeHooks also to "unmarshaling" env variables.