Extensible notion of commands

[stevana]

Currently the type of commands is a simple datatype, with the usual problems of it having to be defined in one file and that its not extensible.

Perhaps a finally tagless approach or something similar to how Hedgehog does it can be adopted to solve this problem?

It's not clear to me how to go about it yet and it will obviously be a big rewrite, so I'm merely opening this to not forget about it and hear other peoples' thoughts.

[stevana]

Closely related to #179.