[#417] Returns 404 for un-handled resources

To use, add ignoredFileExtensions key to static():

```JS
app.use(static({ file: 'tmp/result/index.html', ignoredFileExtensions: /\.\w{1,5}$/ }));
```

Middleware is processed sequentially, so this should be the last call
to catch non-existent asset files. All other paths continue back to
index.html to be handled by the Ember router.

  - Adds another option, ignoredFileExtensions the static middleware
    function, which allows you to specify a regex to capture resource
    paths that should return a 404.
  - Adds a check in static() for ignoredFileExtensions key that tests
    the regex against the request path for a matching file ending.
    If matched, then it sends a 404 error and does not serve the
    index.html file.