Classifiers

[tomodachi94]

Is your feature request related to a problem? Please describe.
We should be able to classify packages based on their licensing, binary/API/both, general categories, etc.

Describe the solution you'd like
Classifiers. The library would have a master unicorn.classifiers submodule with a table that has valid classifiers. Classifiers should be categorized based on their type, eg licenses.

Packages would import the module and state all of their classifiers. Packages hypothetically could invent their own, but this would be discouraged.

cc @znepb, this might affect the UI.