Demonstrate how to make custom commands

[carolynvs]

Right now skeletor shows you how to make essentially a cute wrapper for the exec mixin. But a lot of the power of mixins comes from custom commands. A great example of that is @gaurimadhok 's work with the Docker mixin.

Let's add the plumbing for custom commands and add an example custom command so that when the author is ready to add one, the hard parts are already wired up.

I will hold off on this until we come up with a go 1.18 generics solution for unmarshaling (or something that isn't as difficult as the custom unmarshaling we do today).