Make target to generate shrinkwrap file applying prune

It removes dev dependencies from the local node_modules
and avoid getting those into shrinkwrap file.

HOWTO_RELEASE

@@ -1,7 +1,7 @@
 1. Test (make clean all check), fix if broken before proceeding
 2. Ensure proper version in package.json
 3. Ensure NEWS section exists for the new version, review it, add release date
-4. Recreate npm-shrinkwrap.json with: `npm install --no-shrinkwrap && npm shrinkwrap`
+4. Recreate npm-shrinkwrap.json with: `make shrinkwrap`
 5. Commit package.json, npm-shrinwrap.json, NEWS
 6. git tag -a Major.Minor.Patch # use NEWS section as content
 7. Stub NEWS/package for next version

Makefile

@@ -9,6 +9,12 @@ all:
 clean:
 	rm -rf node_modules/
 
+shrinkwrap: clean
+	rm npm-shrinkwrap.json
+	npm install --no-shrinkwrap --production
+	npm prune
+	npm shrinkwrap
+
 distclean: clean
 	rm config.status*