Add ability to override the extension used on the output files

## Included with this pull request
By specifying the 'generatedExtension' in the config you can have the generated file use a different extension.


## Not included with this pull request
In the future it would be nice to take this further and be able to specify different extensions for different files...

Or have it look for a subextension.  Example:
index.swig -> index.html
index.xml.swig -> index.xml