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.

stylish.sh

@@ -0,0 +1,32 @@
+#!/usr/bin/env bash
+
+set -euo pipefail
+
+function ls-source-files {
+  git ls-files "app/*.hs" "src/*.hs" "test/*.hs"
+}
+
+function check-git-status {
+  [ "$(git status -s '*.hs' | wc -l)" == "0" ]
+}
+
+function stylish {
+  stylish-haskell -i $(ls-source-files)
+}
+
+if check-git-status
+then
+  echo "Running stylish-haskell..."
+  stylish
+  echo "Done."
+  if check-git-status
+  then
+    echo "OK, impeccable style."
+  else
+    echo "KO! Lack of style on those files:"
+    git status -sb
+    exit 1
+  fi
+else
+  echo "git status not clean, aborting"
+fi