feat: cli can take custom alphabet as args

[Phaired]

(Maintainer's note - this is a counterpart to #100 for the CLI tool)

[robertknight]

Thanks for the patch. I requested some adjustment to the wording to make it clear that the alphabet has to match the model. The user can't just use whatever alphabet they like.

In future I think it will make sense to have a way to embed the alphabet in the model file, or provide it as part of a configuration file that is shipped alongside the model. In the interim, adding a CLI argument is useful.

[Phaired]

I agree that embedding the alphabet in the model file would be a good feature.
Sorry for the many commits.

[robertknight]

Sorry for the many commits.

That's not a problem. As a maintainer, I can also do this when merging a PR using the "Squash and merge" option.

For future reference, you can use git rebase --interactive followed by git push --force to tidy up the commit history by editing commit messages, merging commits or other tasks.