Extension: Display inherited metadata

[AriPerkkio]

Is your feature request related to a problem?

tutorial/
├── part
│   ├── chapter
│   │   ├── lesosn
│   │   │   ├── _files
│   │   │   ├── _solution
│   │   │   └── content.md
│   │   └── meta.md
│   └── meta.md
└── meta.md

When opening tutorial/part/chapter/lesson/content.md is open, it's not visible what metadata is inherited from upper levels.

Describe the solution you'd like.

Make the extension display inherited metadata in lower levels as well.

Describe alternatives you've considered.

Additional context

Split from #144.