Add npm script to generate changelog

Author: brianc

package.json

@@ -31,6 +31,7 @@
     "async": "0.2.10"
   },
   "scripts": {
+    "changelog": "npm i github-changes && ./node_modules/.bin/github-changes -o brianc -r node-postgres -d pulls -a -v",
     "test": "make test-travis connectionString=postgres://postgres@localhost:5432/postgres",
     "install": "node-gyp rebuild || (exit 0)"
   },