Add `include_attachments` boolean query parameter

This adds the boolean query parameter `include_attachments` for
`/{db}/_changes` and `/{db}/_design/{ddoc}/_view/{view}` queries.

If both `include_docs` and `include_attachments` equal `true`, the
Base64-encoded contents of attachments are included with the documents
in changes or view query results.

Closes COUCHDB-1923.