Change the CLI of opening haddocks in the browser

I think the behavior in https://github.com/commercialhaskell/stack/pull/2015 is a bit surprising. What if instead:

having `--open` take a package identifier argument (version optional). This argument would implicitly get treated as a target, and we'd open the docs.

Then, we can also have `stack haddock --open-index`, which could work even when the build step has nothing to build.

One reason I like this is that latter is that later on we can further overload it with opening the docs for a particular module, like `stack haddock --open Control.Lens`. That said, I already plan to overload the target syntax for `stack ghci` with something similar. Might consider putting it under `stack ghci --module Control.Lens` or something, though.