add pg option, a simple test and some doc

README.md

@@ -175,6 +175,34 @@ fastify.listen(3000, err => {
 })
 ```
 
+### `pg` option
+If you want to provide your own `pg` module, for example to support packages like [`pg-range`](https://www.npmjs.com/package/pg-range), you can provide an optional `pg` option with the patched library to use:
+
+```js
+const fastify = require('fastify')
+const pg = require("pg");
+require("pg-range").install(pg)
+
+fastify.register(require('fastify-postgres'), {
+  connectionString: 'postgres://postgres@localhost/postgres',
+  pg: pg
+})
+
+fastify.get('/user/:id', (req, reply) => {
+  fastify.pg.query(
+    'SELECT id, username, hash, salt FROM users WHERE id=$1', [req.params.id],
+    function onResult (err, result) {
+      reply.send(err || result)
+    }
+  )
+})
+
+fastify.listen(3000, err => {
+  if (err) throw err
+  console.log(`server listening on ${fastify.server.address().port}`)
+})
+```
+
 ## Development and Testing
 
 First, start postgres with:

index.js

@@ -1,7 +1,7 @@
 'use strict'
 
 const fp = require('fastify-plugin')
-var pg = require('pg')
+var defaultPg = require('pg')
 
 function transactionUtil (pool, fn, cb) {
   pool.connect((err, client, done) => {
@@ -56,6 +56,10 @@ function transact (fn, cb) {
 }
 
 function fastifyPostgres (fastify, options, next) {
+  let pg = defaultPg
+  if (options.pg) {
+    pg = options.pg
+  }
   if (options.native) {
     delete options.native
     if (!pg.native) {

test.js

@@ -117,6 +117,32 @@ test('use native module', t => {
   })
 })
 
+test('use alternative pg module', t => {
+  const altPg = require('pg')
+  t.plan(2)
+
+  const fastify = Fastify()
+
+  fastify.register(fastifyPostgres, {
+    connectionString: 'postgres://postgres@localhost/postgres',
+    pg: altPg
+  })
+
+  fastify.ready(err => {
+    t.error(err)
+    fastify.pg
+      .query('SELECT 1 AS one')
+      .then(result => {
+        t.ok(result.rows[0].one === 1)
+        fastify.close()
+      })
+      .catch(err => {
+        t.fail(err)
+        fastify.close()
+      })
+  })
+})
+
 test('fastify.pg.test namespace should exist', t => {
   t.plan(6)