change default extension of configuration file to json

* renames configuration file when calling ToFile and FromFile
* does nothing if user passes a ```-c``` option to the cli
* does nothing if it would clobber a user's files
* notifies user once change is made

I'm new to go and contributing because I want practice and want to learn. Feedback is welcome.

This should resolve issue #39
We should probably have a constant for this one, too. Maybe `FileLegacy`?