enhancement: subtitles and output-type

[rossmounce]

For the compliance checking use case it's important to know the type of the output.
Some OA policies only apply to outputs of type journal-article whilst there might be slightly different compliance rules for a book-chapter , book , or monograph et cetera...

I know both the subtitle field and the type field are available via the CrossRef API.
Could both of these fields please be represented in the default output from oadoi_fetch ?

Subtitle is useful because often some output titles are just "Introduction" and the subtitle contains the more informative bit... container.title might also be useful to report from oadoi_fetch, although perhaps it was a conscious decision to leave outputs without journal titles?

Or is it simply that the oadoi API does not return these fields? If so I should report this to them, because output type really does matter for this use case!