1.2.0

Version 1.2.0 release!

What's changed.

1. A new addfeed command for adding rss/atom feeds.
2. A new spellcheck (experimental) command for checking spellings in files.
3. Check and update index files on each conversion to remove invalid index links.
4. New optional configs 'site_url', '[ipynb/md]_summary_limit', 'feed_file' available.
5. Add docs for new addfeed, spellcheck and optional configs.

Fixes

1. Remove meta formatting from nbdata files.