Toggle all checkboxes in region with markdown-do or markdown-enter-key

[hmelman]

I often position point just before a gfm checkbox and use RET to toggle the checkbox. It would be nice to be able to toggle all the checkboxes in an active region via RET or perhaps markdown-do. I think both could add a check if the region is active and contains checkboxes to toggle them. I could be wrong but I don't think either currently does something useful with an active region.

• Markdown Mode: 2.6-dev
• Emacs: 27.2 macport
• OS: macOS Big Sur 11.6.4