Add 1 or 2 extra title levels (H4 / H5) to help structure properly the hierarchy of knowledge docs

I find myself rather quickly limited when using Coda for internal knowledge docs, because having only 3 levels of titles is not deep enough even for not ultra-deep/heavy documentation, that I would not want to break out in several pages.

It would be great to add H4 at least, and why not H5.