Add graph feature to show a star chart/graph of a repository

We can use probably a `--graph` option to display a graph of the star/fork/watcher or just the stars of one or more repositories, choosing a specific repository or display a graph for all repos fetched from `search()` is undecided. Play around and experiment with what you should do, if we are going to display a graph for every repo fetched, we should make the `graph` option one of the `--layout` choices.

Don't forget to update the help and usage both in `__main__.py` and also in the readme.