Use haskell-hoogle-command instead of hardcoded "hoogle"

[jappeace]

Obviously it allows the insertion of a custom hoogle command. I now have this server running in a nix-shell per project:

(use-package haskell-mode
  :config
  (custom-set-variables
   ...
   '(haskell-hoogle-command (concat (projectile-project-root) "scripts/hoogle.sh"))
   )
...
)

and that hoogle script:

#! /bin/sh
set -x 

# https://stackoverflow.com/questions/59895/get-the-source-directory-of-a-bash-script-from-within-the-script-itself
DIR="$( cd "$( dirname "${BASH_SOURCE[0]}" )" >/dev/null 2>&1 && pwd )"
ARGS="$@"

nix-shell --run "hoogle $ARGS" $DIR/../shell.nix 
exit 0

It would be slightly more convenient to have this command be a function so it can always return the most recent (projectile-project-root), and do the argument juggling inside lisp instead of a custom script.
But this works for me.