Support for loading environment variables from a configuration file

[mkwan01]

@ThePrez The cdimascio/dotenv-java project already has support for loading env variables from a Java properties file. This is to add the same support to the DotEnv-Java-IBMi project. We will add the following API:

public static synchronized Dotenv loadDotEnv(File configFile) throws DotenvException;

This API will merge the env variables from the config file with the ones from the system. It will persist the env settings in the static instance s_dotEnv,

[mkwan01]

This change is a requirement for the following issue in the AIStream-IBMi project:
ThePrez/AIStream-IBMi#11

[mkwan01]

The change was merged using a fork. No longer need this issue.