Add shellcheck as a pre-commit hook

pre-commit

@@ -0,0 +1,27 @@
+#!/usr/bin/env bash
+
+has() {
+  if (( $# )); then
+    hash "$1" 2>/dev/null
+  fi
+}
+
+is_a() {
+  if (( $# > 1 )); then
+    head -1 "$1" | grep -qE '^#!.*'"$2"'$'
+  fi
+}
+
+find . -name .DS_Store -exec rm -f -- {} +
+
+git status -zu | while read -rd '' xy filename; do
+  if [[ $xy = "??" || $xy = D ]]; then
+    continue
+  fi
+
+  if [[ $filename = *.sh ]] || is_a "$filename" bash; then
+    if has shellcheck; then
+      shellcheck -s bash "$filename"
+    fi
+  fi
+done