fda_orphan_drug plugin migration

[erikyao]

Currently fda_orphan_drug is the only plugin under the plugins folder. It also relies on upstream data processing described in Sander's fda_orphan_drug repo.

Shall we upgrade it into an advance plugin, with all the upstream data processing integrated, and then relocate it to dataload?

Related Issue:

• https://github.com/biothings/mychem.info/issues/96