Add nunaliit dump command line option to exclude attachments

[ahayes]

When looking to extract data from a Nunaliit atlas, it is possible to export the documents as CSV or GeoJSON using the export functionality of the browser-based tools pages, but these formats result in some flattening and reformatting and don't represent the structure in CouchDB as well as Nunaliit's on disk dump format does. Nunaliit's dump format can also be easily re-imported.

When dumping documents with attachments, the dump file and folder structure correctly includes all attachments. This can get quite large. If for whatever reason, these attached files are not required or desired in the dump, it would be nice to be able to optionally dump documents while excluding attachments.