Support custom extension data

[mdoering]

To share other data that coldp does not support out of the box any number of custom csv data files should be supported. To know which they are a new extensions.yml file could be useful. Columns in such files which are foreign keys to name, taxon, type material or references could be identified. But a convention to use {entity_name}_id for such columns could be sufficient and even better, eg name_id, reference_id

Alternately to a new extension.yml file a file name suffix would also work, eg barcode_ext.csv