Reorder tour; a script for reordering

The tour introduction also mentions those tour pages that weren't mentioned before.

A script is included that does renumbering.

```sh
$ ruby renumber.rb --help
Usage: ./renumber.rb [OPTIONS] [-i INDEX | FILES]
    -i, --index FILE                 Use newline-separated list of files
    -e, --encoding ENC               Set character encoding
    -x, --print-index                Show index, don't change any files
    -s, --start-from NUM             Use fresh numbers instead of reorganizing the existing numbers
```

The workflow is pretty straightforward for using this script to
reorganize a multi-file document.

1. Have a clean tree.
2. Use `-x *.md` to print an index; write that to a file.
3. Reorder the lines in the file to match your desired order.
4. Feed the index back in with `-i`.