[#417] Returns 404 for un-handled resources

To use, place a trap404 option immediately before the catchall index
option:

```JS
app.use(static({ trap404: /.js$|.css$/ }));
app.use(static({ file: 'tmp/result/index.html' }));
```

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, trap404 to 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 the trap404 flag that searches the
    request path for a matching file ending. If matched, then it sends
    a 404 error and stops the middleware chain.