Changing publisher cardinality to allow multiple values

It would be useful to change the **publisher** field to allow for multiple agency contributions. This would be useful for datasets that are a result of joint agency collaboration (e.g. many NASA/NOAA datasets) or datasets that are from child agencies (such as HHS/FDA, Commerce/NOAA, etc) where there would be a benefit from listing both agencies as publishers.

Multiple publishers can simply be listed as comma delineated. This may also relate to https://github.com/project-open-data/project-open-data.github.io/issues/89