text: Title: [RFC] Allow dotted environment variables
  Body: **Is your feature request related to a problem? Please describe.**
  Parameters defined in environment variables cannot be accessed with dotted key notation in the same way as parameters defined in settings files, which prevents overriding dotted key parameters with environment variables. 
  
  **Example:** Environment variable `DYNACONF_SERVICE_PORT` cannot be accessed via `settings['SERVICE.PORT']`. Instead, it must be accessed via `settings['SERVICE_PORT']`.
  
  **Describe the solution you'd like**
  One solution would be to use a designated sequence of characters to symbolize a dot separator, possibly two consecutive underscores ( `__` ). This character sequence could be configurable (eg `ENVVAR_DOT_SEPARATOR_FOR_DYNACONF`).
  
  **Example:** Environment variable `DYNACONF_SERVICE__PORT` could be accessed via `settings['SERVICE.PORT']`, and would override any `service.port` parameter defined in a settings file.
  
  **Describe alternatives you've considered**
  An alternative to this could be to simply match either dotted or non-dotted key values to the correlated environment variable. As a note, this is how Spring Framework handles this feature.
  
  **Example:** Environment variable `DYNACONF_SERVICE_PORT` could be accessed via `settings['SERVICE.PORT']` or via `settings['SERVICE_PORT']`, and would override any `service.port` or `service_port` parameter defined in a settings file.
  
  **Additional context**
  This would be especially nice for:
  - allowing natural grouping of related settings (eg `service.host` and `service.port`)
  - allowing overriding of existing parameters defined in a settings file
  - accessing environment variables via object dot notation (eg `settings.SERVICE.PORT`)
  