Add support for extensible records

[samvv]

As described in the README, we'd like our language to support extensible records like languages such as Elm have.

A good candidate for implementing this seems to be by using rows. Thunderseethe's blog has an excellent tutorial on how to use rows, but it will take some time to implement.

[samvv]

@vil02 Given that I've found no way to contact you directly, I will thank you here for showing interest in this project. It has given me the courage to continue working on it!