Add "book package" export and import

[jzohrab]

It would be nice be able to export individual books into files that someone else can load into their Lute. It'd include all the words/definitions, the audio, the bookmarks, everything. I think a feature like that would really help form a Lute community. I know that I'd love to share my texts with other Czech learners.

This would be something like an "anki package" zip file. Work involved:

• define export, import format
• export file versioning, so that importers can deal with old formats
• export book and audio only, no terms?
• export book and terms?
• handle term images
• overwrite existing terms on import, or ignore?
• book with same title allowed or not?

[yue-dongchen]

I was about to create an issue regarding this. These user-generated book "bundles" could indeed be very helpful. I think the inclusion of terms in such bundles would be of trivial benefit, since the central focus is the convenience of sharing books as learning resources. It is analogous to including review statuses of cards in Anki packages – barely useful.