Add mapping key transform hook

This hook allows the user to transform map names (e.g. replace '-' with '_').

(EDITED)
The coresponding code in config/config.go looks like:

```diff
diff --git a/config/config.go b/config/config.go
index 9278ecc..e0e4903 100644
--- a/config/config.go
+++ b/config/config.go
@@ -46,6 +46,16 @@ type CloudConfig struct {
 	NetworkConfig     string   `yaml:"-"`
 }
 
+func init() {
+       yaml.UnmarshalMappingKeyTransform = func(nameIn string) (nameOut string) {
+               return strings.Replace(nameIn, "-", "_", -1)
+       }
+}
+
```
Seems more straightforward to me to make this `var UnmarshalMappingKeyTransform transformString = identityTransform` rather than nil and omit the nil check in `newParser`