Macros

Japaric's branch: https://github.com/japaric/rust-by-example/pull/193. My original PR: https://github.com/japaric/rust-by-example/pull/185. I'm hoping the comments will merge onto the end of his...

Target's helping fix this [comment](https://github.com/japaric/rust-by-example/pull/193#discussion_r15363505). Hopefully it's a little better at showing how repetition works and what is allowed.