Added configuration to add filename as a node on the JSON file

Hello! Thanks for this tool. Love it!

Btw I would like to suggest a change which is to add a node using the file name. The use case applies for the localization. If you have different files for each namespaces in you JSON locale files, merging them may cause conflicts/overrides. So it may be useful to have a parent node in the destination file using the sources filename.

Thanks!