scripts: add a script to run markdownlint check locally

scripts/markdownlint

@@ -0,0 +1,19 @@
+#!/usr/bin/env bash
+
+cd "$(dirname "$0")/.."
+
+if ! program=$(which markdownlint) &>/dev/null ; then
+  program=./node_modules/.bin/markdownlint
+  if ! test -f ./node_modules/.bin/markdownlint; then
+    had_lock=
+    if test -f package-json.lock ; then
+      had_lock=true
+    fi
+    npm install --save-dev 'markdownlint-cli@0.17.0'
+    if [[ -z "$had_lock"  ]] ; then
+      rm -f package-lock.json
+    fi
+  fi
+fi
+
+exec node "$program" "$@"