Since the repository is now mirrored to Tangled, this seems like a
sensible choice, and it might theoretically make it easier for
patches and bug reports to be filed, though in practice that isn't
much of a concern at present.

Debian stable's rustc is too old lmao.

This simplifies the code a fair bit, and there was no good reason
for that limitation in the first place.

Upraded the dependency, and altered the Usage section of the README to
match the new output.

Fixed the capitalization and punctuation, and made the whole thing a bit
more professional.

Renamed the `options` parameter to `choices` to fix some weirdness with
the `clap` help message, and updated the documentation to match.

Also started printing the version number and added a `--version` option.

Added line breaks so it's legible without line-wrapping.