Adds the possibility of serving assets from application's domain

[nunomaduro]

This pull request adds the possibility of serving assets from application's domain.

This is particular useful for developers deploying PWAs using Vapor, as they may want to serve assets directly from their domains such us: your-domain.com/.well-known/assetlinks.json, or your-domain.com/serviceWorker.js.

To get started, developers just need to populate they serve_assets key in the config/vapor.php file:

/*
|--------------------------------------------------------------------------
| Serve Assets
|--------------------------------------------------------------------------
|
| Here you can configure list of public assets that should be
| served directly from your application's "domain" instead
| of the S3 asset bucket or the CloudFront's asset URL.
|
*/
'serve_assets' => [
    '.well-known/assetlinks.json',
    'serviceWorker.js',
],

Note: It's only possible to serve dot files as assets once this pull request gets merged.