Add ability to save and manage notes

Thanks for your work.
For the time being there is no convenient app to create notes with integrated code blocks, links, images, videos for programmers. It would be cool if MacDown had ability to save, search and manage notes.

The issue names no files, tests, or entry points. Start by identifying how MacDown currently opens and saves Markdown documents, then define the scope for note storage, search, and management; done would require an agreed design and corresponding coverage for the requested capabilities.

Written by the indexing model from the issue text.