protoc-gen-doc v0.9

This is the ninth official release.

Changes

• Improve installation instructions for macOS (thanks @guozheng)
• Improve installation instructions for Debian/Ubuntu (thanks @mhaberler)
• Add asciidoc.mustache example template (thanks @ArcEye)
• Don't do HTML escaping in Markdown template (thanks @sunfmin)
• Add support for JSON output