message: Allow setting CARGO_TARGET_DIR
  This ended up being a little more complicated than I wanted because it
  allows for CARGO_TARGET_DIR to be both a relative and absolute path. It
  should work anywhere with bash installed.
diff: #!/usr/bin/env bash
  -(cd blacksmith && cargo build)
  +set -euo pipefail
  -./blacksmith/target/debug/mdbook-blacksmith "$@"
  +TARGET=${CARGO_TARGET_DIR:-target}
  +# https://stackoverflow.com/a/3572105
  +realpath() {
  + [[ $1 = /* ]] && echo "$1" || echo "$PWD/${1#./}"
  +}
  +
  +cd blacksmith
  +cargo build
  +BLACKSMITH="$(realpath "$TARGET/debug/mdbook-blacksmith")"
  +cd "$OLDPWD"
  +"$BLACKSMITH" "$@"
  