Add option to hide an empty TOC

[Tholleman]

I've got the TOC in my template but don't like the look of the warning when there are no headings.
My proposed solution is a new option that hides the TOC in that case

```table-of-contents
hideWhenEmpty: true
```

which would hide the warning:
Table of contents: no headings found

[Tholleman]

Not entirely sure what should happen to the title in this case. I don't use it so I have no strong preference.
If "hideWhenEmpty" is the name of the option then it should also hide the title. If the option is instead named "hideWarning" then you should still render the title.