Add support for guessing project license

[ivan-m]

I can do a PR for this; just not sure if it should go into haskell-cabal.el or haskell-mode.el:

(defun haskell-guess-license ()
  "Guess the license of this project.
If there is no valid .cabal file to get the license field from,
return nil."
  (interactive)
  (when buffer-file-name
    (let ((cabal-file (haskell-cabal-find-file (file-name-directory buffer-file-name))))
      (when (and cabal-file (file-readable-p cabal-file))
        (with-temp-buffer
          (insert-file-contents cabal-file)
          (haskell-cabal-get-setting "license"))))))

One possible change would be to optionally provide the file to act on rather than making it strictly necessary to use it interactively.

[chrisdone]

haskell-cabal seems okay.