Explicit page breaks

[leroycep]

Seems like a useful feature to have, but CommonMark doesn't define any way to create page breaks. This stackoverflow post suggests using some html to do it:

<div style="page-break-after: always;"></div>

We could also try making extra long thematic breaks page breaks. I.E. five or more on a line = page break.

*****

Or two thematic breaks in a row:

***
***

I'm open to discussion on if and how explicit page breaks could be implemented.

[leroycep]

Added in 71cf9a4.

I went for HTML style page breaks. A caveat is that, currently, a pagebreak is only created when style is exactly page-break-after:always;. Any spaces or other style code will be ignored.