Linting integration

This commit brings in an HLint configuration file
and several recommended modifications such as:

* End-of-line extra spaces removal;
* Import lines ordering;
* Redundant $ removal;
* Generalisation of ++ and map to <> and fmap;
* Preferring `pure` over `return`;
* Removing extraenous extensions.

And finally, a `stylish-haskell` helper script
that detects if code files are dirty. Can be useful for CI,
although manually calling it can be nice if you would rather
first implement then beautify.

test/Specs.hs

@@ -1,8 +1,8 @@
 module Main (main) where
 
-import Test.Hspec
 import qualified FediChordSpec
+import           Test.Hspec
 
 main :: IO ()
-main = hspec $ do
+main = hspec $
     describe "FediChord tests" FediChordSpec.spec