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
It's very uncommon to see `if/else` in go. Usually it's simply an `if` without an `else`.

I would probably do:

```go
if err != nil {
  return err
}
fmt.Printf("renamed %s to %s\n", oldPath, currentPath)
return nil
```