Adds the ability to generate haddock documentation for non-inplace
package builds.

In other words this allows

  - `$ cabal new-build --enable-documentation`
  - `$ echo "documentation: true" >> cabal.project && cabal new-build`
  - `echo "documentation: true" >> ~/.cabal/config && cabal new-build`

And it will generate (and install) haddock documentation for any
package.