Menu item to auto-slugify internal links

[jlevy]

Converting internal section links to slugs in Markdown is a recurring pain. And very confusing for certain sections with numbers or punctuation because nuances of slugging rules (see github-slugger).

• Instead of writing [some section](#some-section-blah-blah) let people copy/paste the section heading as [some section](#Some Section Blah-Blah)
• Have a menu item that runs github-slugify on all internal section links that have an uppercase letter or a space in them. (Or it may be safe to slugify all.)
• This would have no effect on all valid slugs.

Bonus features:

• Another option to count all internal links that don't fit the TOC, check for them, and add badlink. to any that are not present in the doc.
• Note this wouldn't handle duplicate sections. Could add a warning message about that.