Improve config system

[whotwagner]

Currently there is a config.yml and there are environment variables. The config.yml is limited to sinatra-context only. It's not possible to use these settings in config.ru. That's why some environment variables can be used to enable some settings for config.ru.

A unified config-system would be great. One that can be used with environment variables and that is not limited to the sinatra-context only. I would suggest something like: https://github.com/rubyconfig/config