justforfunc #35: Implementing the tree command from scratch

Let's go back to basics and implement a well known command using only what the standard library provides. We'll discover the `path/filepath` package with its `Walk` function, but also `os.Stat`, `os.FileInfo`, and much more. It might seem simple, but there's lots to learn! code: github.com/campoy/justforfunc/tree/master/35-tree