Render Haddocks in flag descriptions

[RyanGlScott]

This issue is best illustrated with an example, so take a look at http://hackage.haskell.org/package/accelerate-1.0.0.0#flags:

Sadly, none of the Haddocks are rendered for any of the flags' descriptions, making it quite hard to read. Is it possible for hackage-server to render them?

[hvr]

I see no fundamental problem why we couldn't, as long as Cabal is able to parse the .cabal file

I was planning on improving/refactoring the current haddock markup handling anyway, as hackage-server has been lagging a bit behind Haddock's more recent markup additions (bold-faced, headings); then I could look into making the flag description haddockable as well.