Added the Stackoverflow fetcher: if a query did not return any result, i...

Added the Stackoverflow fetcher: if a query did not return any result, it fetches google with the query and displays the first Stackoverflow result.

Backward compatibility should be ok, but many changes were involved. Among others:
- added a config.rb file for personal API credentials
- added a processor.rb file that takes care of formatting displayed texts in the console
- this involves changes in main: say and created method handle_responses

Readme.md was updated too, with installation details (to get API Key). The `config.rb.example` file has a working API KEY to get an example working straightaway.

Hope you enjoy it, I already love it!