Quit MacDown without saving

[kke]

Slightly related to old #795

Here is how my morning went: I glanced at a project README.md in MacDown and decided to redo it in Vim. Once finished, I took a look at it in again MacDown. Then I quit MacDown and was presented with the question:

• Save
• Revert changes
• Cancel.

I assumed "save" will overwrite and "cancel" is to abort the quitting and "revert" is your typical "discard changes" button. But no. Revert dug up the original file from somewhere(?) and wrote it over the README.md I spent the whole morning writing in vim.