Haddock comments support

The GHC backend needs to fetch the Haddock comments, and put them into the common Translate layer (maybe a bit transformed?), associating them to the correct entity. Then the Kythe frontend should format and emit these.

The Haddocks can be fetched using the Haddock API (has to be opened up, see https://github.com/haskell/haddock/issues/595) based on the GHC AST we already have access to.

Note: a special arg needs to be passed to GHC to have the Doc nodes present in the AST. This is likely [Opt_Haddock](https://downloads.haskell.org/~ghc/7.10.3/docs/html/libraries/ghc-7.10.3/GHC.html), can be passed in GhcApiSupport somehow. See [this GHC test](https://github.com/ghc/ghc/blob/ba597c1dd1daf9643b72dc7aeace8d6b3fce84eb/testsuite/tests/ghc-api/annotations/comments.hs#L31) for inspiration.