Add missing `traverse` method for `GenLocated`

As `Traversable` needs at least one of `traverse` or `sequenceA` to be
overridden.

Signed-off-by: Herbert Valerio Riedel <hvr@gnu.org>
Add test helper
Fixes #231
Fixes #256

We inject -dynamic-too into flags before we run all our actions in the
GHC monad.
Add new field to DynFlags
Fallback to ./resources when Cabal data is not found

(so that themes are found during development)
Fixes #5
Print missing documentation. Fixes #258.