add support for multipart api when fetching a doc

This change return a multipart response `{ok, {multipart, Stream}}` when
using the setting `attachments=true` and passing revs as options with
th the option {`accept. <<"multipart/mixed">>}" in the function
`couchbeam:open_doc/3` . Also this function detect the returned content
type by checking the headers.

With this change you can now efficiently retrieve a doc with all of its
attachments or a doc wit all its revisions.