Add the ability to alter the filenames of the merged JSON files

[bmds]

When you have JSON files contained within multiple subdirectories the array keys for the sets of data become awkward. It would be nice to have the ability to pass a function to alter the names of these keys to something more appropriate.

I've updated the second argument to accept either a function or an object. Exiting implementations will continue to work as they do. However you could now call

jsoncombine(filePath, {
  dataConverter: function(data){...},
  pathConverter: function(data){...}
});

To pass either function independently & have the plugin fall back to a default if neither are passed.