Add post-decode hook

[janslow]

Hi, thanks for the work you've done on this project.

One thing I've been struggling to do is run a function on the decoded value. I've got two use cases for this:

1. Call an Initialize() function on any type that implements it, after decode has been completed (to apply custom configuration)
2. Call a Validate() error function on any type that implements it, to perform custom validation.

Note, I want these to be performed on nested types (e.g., after type Foo struct { Bar Bar } is decoded, Bar.Validate() should be called if it exists), as well as allowing the parsing of Foo to be interrupted (i.e., if Foo.Bar.Validate() returns an error, the decoding of Foo should be be considered a failure, so mapstructure shouldn't try to decode any other fields.

Proposal

I could imagine mapstructure providing something like the following API to cover these use cases:

cfg := mapstructure.DecoderConfig{
	PostDecodeHook: mapstructure.ComposePostDecodeHookFunc(
		mapstructure.PostDecodeHookFunc(func(value reflect.Value) error {
			if v, ok := value.Interface().(Validatable); ok {
				return v.Validate()
			}
			return nil
		})
	),
	Result: &output,
}

This would be called at the end of the mapstructure.Decoder.decode() function, right before the final return statement.

Does this seem like a sensible enhancement? If so, I'm happy to put together a PR to implement it, if you'd consider reviewing it.

[davidcassany]

This is something I also have been wondering about. I am using mapstructure in conjunction with viper to load relatively complex yaml files into structs and having a hook to apply some logic right after the unmarshal is done would be really helpful to run some sanity checks and also prevent some co-occurrence issues. Obviously I can do the same as an extra step after unmarshaling, however having a chance to tie it all together at mapstructure level would be really nice.