Adding a way to get env vars from Settings class

[marcieltorres]

What changes do you are proposing?

Improving the Settings class to handle environment variables.

Before

from config.settings import settings
from os import getenv

app_name = settings.get('app_name')
env_var_value = getenv('env_var_name')

After

from config.settings import settings

app_name = settings.get('app_name')
env_var_value = settings.get_from_env('env_var_name')

1. When the settings package is already imported, the os import becomes unnecessary;;
2. The Settings class is now the primary interface for accessing both environment and static settings values;

How did you test these changes?

I created unit tests to verify the new behavior.