[Feature Request]: post cover

[roberto-butti]

Now, there are:

• cover (if exists) (image or html)
• TOC
• content (pages from markdown)

I would like to suggest having an optional "post cover" after the cover and TOC. This may be useful for having the typical page with inscription "thanks to..." or some quotes.

My proposal is:

• if exists a /assets/post-cover.html file (with some content, like cover.html) (so not mandatory), append the post-cover.html file after the cover in buildPdf() function.

If you want, I can open a PR for this.