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
Do you think it's important to tell the user we renamed the file ? Perhaps it's sufficient to tell them the name that the config was finally written to ?