Add "Hello World" example as DUNE extension.

[mikelik]

Plugins are in src/plugin directory.

For each plugin a subdirectory has to be created with main.py script.

Each main.py needs to implement functions:

• handle_args
• add_parsing
• optionally: set_dune(dune) - if the plugin wants to use DUNE's API.

[mikelik]

Closing, this was only an example.