Warn when a yaml file has duplicated keys

Try using `UnmarshalStrict`, if it fails, print the errors as a warning and try again with plain `Unmarshal`.

Errors look this way:
```
2019/03/26 15:11:33 warning reading config file: yaml: unmarshal errors:
  line 6: key "api_key" already set in map
```

viper.go

@@ -1462,9 +1462,13 @@ func (v *Viper) unmarshalReader(in io.Reader, c map[string]interface{}) error {
 
 	switch strings.ToLower(v.getConfigType()) {
 	case "yaml", "yml":
+		// Try UnmarshalStrict first, so we can warn about duplicated keys
+		if strictErr := yaml.UnmarshalStrict(buf.Bytes(), &c); strictErr != nil {
 			if err := yaml.Unmarshal(buf.Bytes(), &c); err != nil {
 				return ConfigParseError{err}
 			}
+			log.Printf("warning reading config file: %v\n", strictErr)
+		}
 
 	case "json":
 		if err := json.Unmarshal(buf.Bytes(), &c); err != nil {