Asciidoctor: Add TODO comment style

[McPringle]

I use Asciidoctor to write books and I often add a TODO somewhere. If I add it as a comment, it will not be visible in the HTML oder PDF output. So I just write in admonition style: TODO: fobar. Doing so it will be visible in the output but it looks like regular text. I would suggest to add a TODO admonition like this:

TODO: foobar

Or, if the admonition style would lead to confusion, a special TODO comment handling like this:

// TODO foobar

A TODO should be visible in the output and easily be recognizable, e.g. an intensive yellow background.

[mojavelinux]

This topic has come up in #1692.

If you want something like this today, you can use the highlight / mark element.

#TODO: foobar#

That's what @graphitefriction and I use when we write.

[mojavelinux]

For more information about the highlight element, see https://asciidoctor.org/docs/user-manual/#custom-styling-with-attributes