Add ghc-prim package to dependencies and bump version to 2.0.1

If the ghc-prim package is not there it complains with a

src/Text/Email/Parser.hs:22:8:
    Could not find module `GHC.Generics'
    It is a member of the hidden package `ghc-prim'.
    Perhaps you need to add `ghc-prim' to the build-depends in your .cabal file.
    Use -v to see a list of the files searched for.