Allow Windows config file to be read from other locations

In reference to issue #149 it would be nice to be able to have an entirely portable version of Exercism by ensuring that the config file, which on Windows is currently placed in `C:\Users\[username]\AppData\Roaming\exercism\user.json`, can be read from either the same folder where the `exercism.exe` executable is located, or by allowing a `--config` parameter on the `configure` command allowing the user to specify their own location for the `user.json` file.

Defaulting to attempting to read the `user.json` file from the same folder as the `exercism.exe` executable and falling back to then searching the user's `AppData` folder (as above) would be a great way to allow all exercism-related files to be stored within the same top-level folder, allowing for a truly portable version of Exercism.