Micropub: add in DailyMed to supported/challenged by

[rkboyce]

Currently, the Micropublication plug-in supports linking evidence items indexed in PubMed via the "supported by/challenged by" interface. Some drug information is often found in the product label and not in the scientific literature. Thus, a possible requirement for using micropubs in drug evidence curation would be to include DailyMed as a search source.