Enable Marshalling to string

[bricef]

I'd like to marshall the config to a string (or to a file-like object) so that it can be written to stdout or used in some other way.

This isn't currently allowed as the Marshal functions are private.

[bricef]

This can be solved by using the marshalling the entire config manually. for example:

c := viper.AllSettings()
bs, err := yaml.Marshal(c)
if err != nil {
    // handle marshalling error
}
fmt.Print(string(bs))