Add support for custom struct tags via environment variable

[NickYadance]

Fix: #234

Add support for custom struct tags via environment variable, set gotags to read by system env variables.

To easily reference field by custom go tags like json, mapstructre, which is especially useful when dealing with generated go structs like protobuf.

[NickYadance]

@antonmedv how do you think about setting custom gotag from system env ?

[antonmedv]

Why do we need to specify this from env?)

[NickYadance]

I tried to set gotag in config like expr.Compile("", expr.WithGotag("json")), but there is some expr usage that cannot reach the config value, like in lib.go. Even in where we can reach the config value, it feels code intrusive to pass the value all the way down there. So i picked env since it is easy to read.

expr/builtin/lib.go

Lines 420 to 432 in 1c09e5e

	case reflect.Struct:
	fieldName := i.(string)
	value := v.FieldByNameFunc(func(name string) bool {
	field, _ := v.Type().FieldByName(name)
	switch field.Tag.Get("expr") {
	case "-":
	return false
	case fieldName:
	return true
	default:
	return name == fieldName
	}
	})

[antonmedv]

Better not introduce another configuration surface and use same conf pgk.