Add support for configuration files

[chahuistle]

We are expanding our Java-based toolset, so it might make sense to use a generic way to read configuration files.

In order for this to be generic enough, default configuration files should be named after the tool they are related to. For instance, a tool named nexus-listener-service should have its default configuration file under ~/.nexus-listener-service.config.

To make things simple, we can make all config files to be yml files and implement this as part of the ToolExecutor mini framework. The framework would read configuration files upon tool start-up, use the annotated ...Command class to map each entry in the configuration file (if any) to its corresponding picocli parameter to create an instance of an AbstractCommand. Parameters given via the command line would of course override values found in the configuration file.