Support config options in preload

[danielfsousa]

We should support command line arguments and environment variables to pass options to config() method when preloading dotenv-azure, like dotenv.

Also, the preload script config-safe should be removed.

Before:

node -r dotenv-azure/config-safe your_script.js

After:

node -r dotenv-azure/config your_script.js dotenv_azure_config_safe=true

# or

DOTENV_AZURE_CONFIG_SAFE=true node -r dotenv-azure/config your_script.js