add KEGG disease id mappings

some KEGG ids have been added to the disease ontology, but i think they might be generated through round-about methods.

here's the direct mapping file, along with their hierarchy:
http://www.kegg.jp/kegg-bin/download_htext?htext=br08402.keg&format=htext

note that the ids in the format 'H00042' are the disease ids, if prefixed with ds, like "ds:H00042"

also see github ticket: https://github.com/monarch-initiative/disease-miner/issues/2