`stack hoogle` command

[chrisdone]

I want this! It should:

1. Run hoogle with the right database according to the stack config in the directory in which it's run (so it's easily ran from a terminal or by IDEs).
2. Automatically download the hoogle database from stackage.org e.g. https://www.stackage.org/snapshot/lts-2.1/db.hoo for lts-2.1.
3. Put it in some reasonable place like ~/.stack/hoogle-databases/lts-2.1.hoo.
4. Or when running inside docker, use the hoogle database provided by the docker.

The last docker point means we probably need to sync up some things.

With this command I could provide some nice Emacs support with a live hoogle search buffer, and I'd basically never have to leave Emacs to open my browser to read docs again. This closes off two big pain points for me: (1) always having to open the browser and leave my editor environment, (2) needing an internet connection to view docs. This will also basically make my whole haskell-docs package redundant which I'll be able to deprecate in favor of stack.

Pinging @manny-fp and @snoyberg for feedback/opinions.

[chrisdone]

Second thoughts, seems hoogle's -d expects a directory. So it'd probably be better as:

~/.stack/hoogle-databases/lts-2.1/lts-2.1.hoo

This has some potential for putting other databases in there, such as ones generated from your local projects, which could be neat for megarepos.

[ndmitchell]

We should probably be careful about doing too much work with Hoogle 4, when I want to push forward on Hoogle 5, which changes most of this stuff quite dramatically. It's a bit of an awkward middle situation though.