Introduce a pre-cobra CLI project

[illicitonion]

I think we should add a no-cobra no-dependencies project which exercises:

1. stdout vs stderr
2. Reporting errors to the user (i.e. having a main wrap an error-returning function and separating concerns of user output vs functionality
3. Intro to testing CLI app components
4. Implementing an interface on a novel struct

Possibly this could take the form of two exercises along the lines of:

1. Write all args comma-separated to a file
2. Implement your own version of bytes.Buffer (or a slightly cut-down API thereof)

[illicitonion]

#116 covered 1 and 2, and maybe 3 here probably sufficiently. Implementing interfaces, we haven't done - maybe the suggested "implement bytes.Buffer" would be a decent additional small project.

[illicitonion]

#130 fixed up the interfaces part 🎉