This introduces a new `--start-at` CLI argument and corresponding
`start_at()` method on the Exporter type that allows exporting of only a
given subdirectory within a vault.
See the updated README file for more details on when and how this may be
used.
This removes mdBook in favor of a flat docs folder generating a single
`README.md` in the repository root.
Installation and usage instructions have also been expanded slightly.
