Implements #23, report which config files were loaded

I've also run into a situation where it would be useful to report which configuration files were actually loaded by rc, so have added support for a `_rcfiles` property to be added to the returned configuration object which contains the list of files, along with tests and a section documenting it in the readme.

It works by passing an array into the json util method - when it parses a json/ini file, it adds the name of the file to the list.  The list is then added to the configuration object as a non-enumerable property.

I hope this is helpful, please let me know if you have any comments.