Mark `ComrakParseOptions` and similar `#[non_exhaustive]`

[Trevor Gross]

This will allow for adding more configuration fields in the future without breaking compatibility (it seems like some issues may be requesting this). However, this itself would be a breaking change; users would need to change from e.g.

let opts = ComrakParseOptions {
    smart: true
    default_info_string: None,
    relaxed_tasklist_matching: true,
}

to:

let opts = ComrakParseOptions {
    smart: true
    default_info_string: None,
    relaxed_tasklist_matching: true,
    ..Default::default()
}

[Trevor Gross]

Ah, this is actually handled in #305