Allow checking off checkboxes in markdown

Copyparty is able to render markdown files, and even renders checkboxes (well, it shows a green `Y` when checked, and a red `N` when unchecked, but that is no problem as far as I am concerned).

I'd like to be able to (just like, for example in GitHub) be able to click the checkboxes to check off an item. Currently, the only way (seemingly) to check off an item is to edit the markdown file and place an `x` in the text.