Handle document attachments

[adrienverge]

See documentation at https://docs.couchdb.org/en/stable/api/document/attachments.html

I choose not to cache data in the class, contrary to Document objects, because attachments can be very large.

For that reason, and for simplicity, file contents are returned directly from Attachment.fetch(), and are sent directly to .save().

The API currently looks like this:

att = doc.attachment("file.webp")
await att.save(binary, "image/webp")

att = doc.attachment("other.html")
data = await att.fetch()
content_type = att.content_type

await att.delete()

Criticism and good ideas are welcome!

Closes #18.

[adrienverge]

Thanks for the fast review. I've updated the commit accordingly.

IIRC content_type is a member variable, so no need for await?

Sure, sorry, it was a typo.