add optional validateStatus fn property

If `validateStatus` property is provided in the
config and set to a function, it will use this to
validate whether a status is valid.

The default validateStatus if not provided is

```js
validateStatus: function (status) {
  return status >= 200 && status < 300;
}
```

To also allow a 401 unauthorized as a valid status

```js
validateStatus: function (status) {
  return status === 401 || (status >= 200 && status < 300);
}
```

README.md

@@ -183,6 +183,9 @@ var opts = {
   headers: {
     'x-custom': 'headers',
   },
+  validateStatus: function (status) {
+    return status >= 200 && status < 300; // default if not provided
+  },
 };
 
 // Usage with callback function

lib/wait-on.js

@@ -29,6 +29,7 @@ const WAIT_ON_SCHEMA = Joi.object({
   reverse: Joi.boolean().default(false),
   simultaneous: Joi.number().integer().min(1).default(Infinity),
   timeout: Joi.number().integer().min(0).default(Infinity),
+  validateStatus: Joi.function(),
   verbose: Joi.boolean().default(false),
   window: Joi.number().integer().min(0).default(750),
   tcpTimeout: Joi.number().integer().min(0).default(300),
@@ -271,7 +272,7 @@ function createHTTP$({ validatedOpts, output }, resource) {
     : { url };
   const socketPathDesc = urlSocketOptions.socketPath ? `socketPath:${urlSocketOptions.socketPath}` : '';
   const httpOptions = {
-    ...pick(['auth', 'headers'], validatedOpts),
+    ...pick(['auth', 'headers', 'validateStatus'], validatedOpts),
     httpsAgent: new https.Agent({
       rejectUnauthorized,
       ...pick(['ca', 'cert', 'key', 'passphrase'], validatedOpts),

test/api.mocha.js

@@ -81,6 +81,28 @@ describe('api', function () {
     });
   });
 
+  it('should succeed when custom validateStatus fn is provided http resource returns 401', function (done) {
+    var opts = {
+      resources: ['http://localhost:3000'],
+      validateStatus: function (status) {
+        return status === 401 || (status >= 200 && status < 300);
+      },
+    };
+
+    setTimeout(function () {
+      httpServer = http.createServer().on('request', function (req, res) {
+        res.statusCode = 401;
+        res.end('Not authorized');
+      });
+      httpServer.listen(3000, 'localhost');
+    }, 300);
+
+    waitOn(opts, function (err) {
+      expect(err).toNotExist();
+      done();
+    });
+  });
+
   it('should succeed when http resource become available later via redirect', function (done) {
     var opts = {
       // followRedirect: true // default is true