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
Per the golang style guide, all exported functions should be commented (see the style of the other exported comments in this package). For now I've only been trying to clean up the `config` package–`main` has larger issues than style at the moment.

http://golang.org/doc/effective_go.html#commentary

If this method is not used from the outside, it can be unexported.