Allow setting additional configuration options.

Currently when you set one configuration option, it blows away previously set configuration options.

If we set a custom directory, then the value is set.

    $ exercism configure --dir="~/code/exercism"
    The configuration has been written to /Users/josh/.exercism.json
    Your exercism directory can be found at /Users/josh/code/exercism

    $ cat ~/.exercism.json
    {"apiKey":"","exercismDirectory":"/Users/josh/code/exercism","hostname":"http://exercism.io"}

However, when we set a key, the custom directory is blown away

    $ exercism configure --key="abc123"
    The configuration has been written to /Users/josh/.exercism.json
    Your exercism directory can be found at /Users/josh/exercism

    $ cat ~/.exercism.json
    {"apiKey":"abc123","exercismDirectory":"/Users/josh/exercism","hostname":"http://exercism.io"}