tasty-discover refactor

Created by: lwm

Since this was a straight fork and hack of hspec-discover, the code organization could be improved to better fit the purpose of this library.

The plan is then to:

• split tasty-discover into 5 modules
  • Config.hs, Parse.hs, Run.hs, Type.hs and Util.hs.
• roll a project Prelude which includes all the necessary imports
  • Not really sure if this is a good idea, but it might ease development effort.