Save on Blur

It would be lovely if there was a Preferences setting to autosave the document on blur (when we switch away from the MacDown app to another app)

In other applications where I've seen this feature, there were a couple options:
A. Default (no autosave to original file, but macOS standard behavior)
B. Autosave on an interval - eg. every 5 mins
C. Save on blur (instantly)
D. Save after N seconds in the background.

Markdown is often used by developers/bloggers/people using git or other external version control tools, so saving to disk is important if the user might be switching over to a terminal or GUI version control tool, and may not remember to have hit save manually.

Option C is what I'm asking for for my use case, but I could see Option D being valuable if you have automated tooling that will export things / reload your website, and you might not want to always autosave if you're often tabbing over to your browser to look up something and then coming back. In that case, Option D allows the user to customize the behavior to fit their workflow -- avoiding unnecessary work but still getting the benefits of autosave.

## Future Directions

Often with working with renderable formats (like markdown), I use a couple applications at once, and the most enjoyable tools also watch the file for external modifications -- that way they're collaborating with each other. -- Not sure if we want to consider that related to this feature request, but if we're autosaving, we probably should also consider autoloading from disk on external changes!

Start by reviewing MacDown’s Preferences and document save/blur behavior to determine how the requested autosave modes fit together. The issue does not name files or tests; define whether the initial scope is save-on-blur only or also interval, delayed, and external-change handling, then verify the chosen behavior in the macOS app.

Written by the indexing model from the issue text.