Add config loader from config file

[eSoares]

With this config we load configuration variables from a config file and from the environment. If a variable is present in both (config and env) the env variable is used. This makes easy to quick change a particular variable between restarts (the debug level e.g.).

[eSoares]

Just to add to the idea of loading tokens/vars from a config file. Each plugin could have it's own section with their parameters, something like:

.....
[plugin_weather]
token = ....
default_city = ...