Added environment_nested plugin

[thorstenhuhn]

Adding environment_nested to the list of datasources and setting environment variable restapi_server_port=8080 will overwrite value 80 from defaults.

data_sources: [ "defaults", "file", "environment_nested" ]
template_sources: [ "file" ]

defaults:
  global:
    restapi:
      server:
        port: 80

[markround]

Awesome! Thanks for the great PR :)