Feature idea: Render gfm checkboxes as widgets

[hmelman]

Would be neat to render gfm checkboxes as actual widgets (from the emacs widget package used in customize) such as toggle or checkbox when markdown-toggle-markup-hiding is enabled.

[hmelman]

I've been doing the following in my markdown-mode-hook and it's been great. Clicking with the mouse or hitting RET with point in them works seemlessly.

(push '("[ ]" . ?🔲) prettify-symbols-alist)
(push '("[x]" . ?✅) prettify-symbols-alist)
(prettify-symbols-mode)