couchdb: Add multipart support to BulkGet endpoint

[flimzy]

In CouchDB 2.3, /_bulk_get has multipart/mixed support, to download attachments. Adding support for this could be nice.

Some possible implementation details:

To trigger this mode, perhaps the CouchDB driver could interpret kivik.Options{"attachments": true} to trigger this behavior. This retains near semantic parity with other functions, since the application/json response only contains attachment stubs.

Then to access these attachments, perhaps the Rows interface could be given an Attachments() *Attachments method, to match the Attachments field on the Row type.

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Migrated from go-kivik/couchdb#141