Add `format=source` parameter, implemented for resources

See https://github.com/lobid/lodmill/issues/478

Returns Aleph-MAB-XML for lobid-resources. Expects hbz01 index on
the same Elasticsearch cluster as used for the other queries.

Index is created with https://github.com/hbz/mabxml-elasticsearch
Merge pull request #36 from hbz/issue-478-mabxml

Add `format=source` parameter, implemented for resources