Add support for parts

[crowdagger]

Currently the top level headers are chapters. But sometimes chapters are separated into multiple "parts" (or "books" or "episodes" or whatever). It could be nice to have an option for this.

I was thinking maybe a directive in the chapter lists, eg instead of

+ chapter_1.md
+ chapter_2.md
+ chapter_3.md

+@ The first part (numbered)
+ chapter_1.md
+ ...
-@ Unumbered part
+ chapter_n.md
...

But it's a bit ugly. Plus I'm not sure exactly of the various usages (a part should probably generate a page on its own, but is it enough to have a name, as for chapter's titles, or should it also have a way to include arbitrary markdown?)

[crowdagger]

I didn't point it in the commits' messages as I had no connection when I wrote them, but it has been fixed today ^^