Lektor-atom

Replace Werkzeug's [Atom](https://werkzeug.palletsprojects.com/en/0.15.x/contrib/atom/)
with Pelican's Django-based [feedgenerator](https://github.com/getpelican/feedgenerator)

- Pin werkzeug to be able to use the atom feed.

- Ensure universal builds
- Bugfix in build program
- Better README

- Python 3 compatibility (thanks to Dan Bauman).
- colored error output during build, fix for Markdown-formatted item subtitles.

- Initial release.