Support for base URL

[coder8338]

I'm trying to add this to the script at https://github.com/coder8338/appbox_installer for coder8338/appbox_installer#3

However the URL needs to be http://0.0.0.0:5000/deemixrr as we're using an nginx proxy and cannot create subdomains.

Using Kestrel__EndPoints__Http__Url=http://0.0.0.0:5555/deemixrr results in System.InvalidOperationException: A path base can only be configured using IApplicationBuilder.UsePathBase().

Is this possible or a feature you would consider?

[TheUltimateC0der]

Feel free to make a PR, I will be happy to merge it.