Add pre-commit hook script to run nixfmt check.

hooks/pre-commit

@@ -0,0 +1,22 @@
+#!/usr/bin/env bash
+
+printf "%s\n" "Running nixfmt pre-commit hook.";
+if ! which grep >/dev/null; then
+  printf '%s\n' "ERR: grep not found.";
+  exit 1;
+fi;
+
+if ! which nixfmt >/dev/null; then
+  printf '%s\n' "ERR: nixfmt not found.";
+  exit 1;
+fi;
+
+RESULT=0;
+for file in $(git diff --name-only --cached); do
+  if grep -q "\.nix$" <<< "${file}"; then
+    nixfmt -c <<< "$(git cat-file blob :"${file}")" 2> >(sed "s|<stdin>|${file}|");
+    RESULT=$(( RESULT || $? ));
+  fi;
+done;
+
+exit ${RESULT};