Add haddock-style-module configuration

[brandonchinn178]

Blocked on #49?
Resolves #74

Adds a haddock-style-module option that specifies the haddock style to use specifically for the module docstring (when not set, resolves to whatever haddock-style is — the current behavior)

Tested with this file:

{- |
Module: Foo
-}

module Foo where

-- | This is a
-- multi line
-- haddock
foo :: Int
foo = 42

and this command:

$ stack exec -- fourmolu --haddock-style=single-line --haddock-style-module=multi-line Foo.hs

{- |
Module: Foo
-}
module Foo where

-- | This is a
-- multi line
-- haddock
foo :: Int
foo = 42

[brandonchinn178]

Orthogonal, but related, to #130

[brandonchinn178]

Checklist:

• CHANGELOG.md has been updated
• Configuration docs in README.md have been updated
• fourmolu.yaml updated to stay in sync with config in README.md

[joncol]

Hi @brandonchinn178! Any chance this might see a revival now that #49 is resolved?

[brandonchinn178]

Yup! I was wondering if anyone else wanted this or if it was just me 😛 I'll merge soon