Pelican 4.1

• Live browser reload upon changed files (provided via Invoke task)
• Add pyproject.toml, managed by Poetry
• Support for invoking python -m pelican
• Add relative source path attribute to content
• Allow directories in EXTRA_PATH_METADATA
• Add all_articles variable to period pages (for recent posts functionality)
• Improve debug mode output
• Remove blank or duplicate summaries from Atom feed
• Fix bugs in pagination, pelican-import, pelican-quickstart, and feed importer