Bad requirement of MultiParamTypeClasses

``` haskell
{-# LANGUAGE FlexibleContexts #-}

module Test where

import Control.Monad.Reader

test :: MonadReader a m => m a
test = undefined
```

This code is OK for GHC, but stylish-haskell requires additional MultiParamTypeClasses.

The error is `Language.Haskell.Stylish.Parse.parseModule: could not parse a.hs: ParseFailed (SrcLoc ".hs" 7 28) "MultiParamTypeClasses language extension is not enabled. Please add {-# LANGUAGE MultiParamTypeClasses #-} pragma at the top of your module."`

UPD. Depends on https://github.com/haskell-suite/haskell-src-exts/issues/304