* Implement `hx --tutor` and `:tutor` to load `tutor.txt`
* Document `hx --tutor` and `:tutor`
* Change `Document::set_path` to take an `Option`
* `Document::set_path` accepts an `Option<&Path>` instead of `&Path`.
* Remove `Editor::open_tutor` and make tutor-open functionality use
`Editor::open` and `Document::set_path`.
* Use `PathBuf::join`
Co-authored-by: Ivan Tham <pickfire@riseup.net>
* Add comments explaining unsetting tutor path
Co-authored-by: Ivan Tham <pickfire@riseup.net>
