Add ngx-env.js versioning

[VitalyEmelyanov]

Thanks for the great project!

When using @ngx-env/builder, the final index.html is injected with <script src="/ngx-env.js"> line, which has a static name, and it can lead potentially to the caching issues.

It would be great to add a special optional env variable that can be used during build to output something like <script src="/ngx-env.js?v=1.2.3">. That will ensure the file does not have the caching issues.