Address this issue: merge pelican-open_graph plugin into SEO plugin.

What's Changed

• Improve content rewriting by @patrickfournier in #77
• Enhance OpenGraph tags by @patrickfournier in #78
• Add OpenGraph tags for Article content by @patrickfournier in #79

New Contributors

• @patrickfournier made their first contribution in #77

Full Changelog: 1.4.0...1.5.0