Allow setting additional configuration options.

[jish]

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"}

[kytrinyx]

Thanks for the contribution! I don't see that the new AddValues method is being called. Is this a work in progress, or would you like me to pull this in and then change the handler to use it?

[kytrinyx]

Actually, I have a few moments to finish this, I'm pulling it in. Thanks ❤️

[jish]

Yea, I submitted it before I actually used the AddValues method :x I realized that this morning, thanks for cleaning it up! :)