Support newer Snafu

The Snafu version this drags along for the ride is a few versions behind. That's making it clunky to try to use with my crate that has both a CLI bin and a library. So I had to basically re-implement the bits I needed.

The issue mentions the Snafu dependency and compatibility with crates that contain both a CLI binary and a library, but names no files or tests. Start by locating the dependency declaration and the CLI/library integration points; done means newer Snafu versions work without reimplementing the required behavior.

索引モデルが issue の本文から書いたものです。