auto reload the markdown file when other app edit it

when editing file, I like use vim.

when I have edited a file, the macdown cannot reload the edited file automatically.

Use the issue as the starting point and reproduce it by opening a Markdown file in MacDown, editing that file in Vim or another application, and observing whether MacDown updates. Done means an externally edited open file reloads automatically without losing the expected document state.

Written by the indexing model from the issue text.