Implement reflection based en/decoding for map[string]interface{}

[klauspost]

Especially the decoder could benefit from an internal map[string]interface{} decoder, so we don't have to parse the json, then hand it to json.Unmarhal (which of course re-does the parsing).

[robx]

This seems related to #115, though fixing the issue there would require an internal interface{} decoder if I understand correctly.