Change the CLI of opening haddocks in the browser

[mgsloan]

I think the behavior in #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.

[silky]

opening via package identifiers sounds fantastic to me; it will be really amazing to get to the haddock docs for any installed package so quickly, especially, as mentioned in another thread, the doom that is the current hackage docs.

[mgsloan]

@silky Yup, it's pretty great! As of today, you can now do stack haddock --open pkg to open the docs for that package, but you have to specify just that single package as a target.