Updated cheerio to latest stable.

Reason: https://github.com/cheeriojs/cheerio/issues/52
Revert "Updated cheerio to latest stable."

This reverts commit 102bb09424aaeb777462c11f3ba50d5f46ea59b8.
Made "cheerio-cli" a separate image so I don't have to be careful with updating the cheerio-deps.
Actually, including the main script in the ENTRYPOINT makes more sense.

Also removed an ADD accidentally left behind.
Added the "listings" package to defaults.
Initial (broken) version of a slightly less crappy gdochtml2tex.
Added support for more formattings etc.
Worked out a few wrinkles (suppressing titles, superscripts and doc-internal links, and normalizing some special chars).
Added some links to README.
Added support for footnotes.
Added support for forced newlines.
Added support for numbered lists.
More special chars special treatment...
Added support for document-internal links.