Viper has a Set(key string, value interface{}) method, which allows for easy updating values.

Koanf does not, thus leading to code like this all over the place:

_ = k.Load(confmap.Provider(map[string]interface{}{key: value}, "."), nil)

It would be nice to be able to set individual values so that we can simply do:

k.Set(key, value)

I'd be happy to raise PR if you're okay with it.