Implemented environment specific configuration file loading.

[Aodai]

This pull request implements the suggested features in #14

[Nexitisyo]

I think its missing the setting of an active environment via CLI.
Maybe a new command?
$ set env
Not sure if the application is already smart enough to swap environments at runtime. Maybe we need to reboot the CLI after that?

[Aodai]

It reads the environment from the DOTNET_ENVIRONMENT env variable as explained in the Discord channel which is the default behaviour for the generic host builder.