Fix ob-haskell.el to work with custom ghci prompts
* lisp/ob-haskell.el (haskell-prompt-regexp): Define defvar
`haskell-prompt-regexp`. This variable will override the variable set by
inf-haskell, when `org-babel-execute:haskell` is called.
(org-babel-execute:haskell): Make sure that
`comint-prompt-regexp` is set appropriately to enable correct parsing
of "λ"-prompts. Set `comint-preoutput-filter-functions` appropriately to
enable correct parsing of coloured ghci prompts.
The problem was that code sent back from the inf-haskell buffer to
org-babel wasn't parsed correctly in `org-babel-comint-with-output`.
This occured when the user uses the commonly used "λ"-prompt.
TINYCHANGE