staticsite news: github mode and post series

pdo staticsite sw eng

GitHub mode

Tobias Gruetzmacher implemented GitHub mode for staticsite.

Although GitHub now has a similar site rendering mode, it doesn't give you a live preview: if you run ssite serve on a GitHub project you will get a live preview of README.md and the project documentation.

Post series

I have added support for post series, that allow you to easily interlink posts with previous/next links.

You can see it in action on links and on An Italian song a day, an ongoing series that is currently each day posting a link to an Italian song.