Support more book divisions

[Omikhleia]

Closes #19

Divisions

Slightly different from the discussion in the issue, after more thoughts and tests:

• frontmatter
  • No part and chapter numbering (hard-coded, this is the semantics of this division)
  • Roman folios (default styling, configurable)
• mainmatter
  • Part and chapter numbering (default styling, configurable)
  • Appendix numbering (default styling, configurable) - see below
  • Arabic folios (default styling, configurable)
• backmatter
  • No part and chapter numbering (hard-coded, this is the semantics of this division)
  • Appendix numbering (default styling, configurable) - see below
  • Arabic folios (default styling, configurable)

Moreover:

• Divisions always start on an odd page, that's the only consistent way with page number reset.
• Page number reset occurs if the display format changes between two division (e.g., with the default styles, after the front matter)
• All divisions accept parts (and it's up to the user to be consistent with his own expectations here)
• Layout changes are not considered (on a second though, they require a different approach, orthogonal to these divisions)
• As noted, front/back always have unnumbered parts/chapters, by their very definition. It seems fairly ok to me, and it avoids introducing a whole additional set of inherited styles with potential inconsistencies)
• I can seen an interest in authorizing parts everywhere for now

Appendix sub-division

Likewise, after some tests, I think this is acceptable:

• May occur only once either in the mainmatter or backmatter in a given part. (Changed here: if we can have chapters in all parts, then we can also have appendices in all parts for a more general case.)
  • So once in a book without parts
  • Once in a part in books with multiple parts
• Is always numbered by default even in the backmatter (default styling, configurable)
• Resets the chapter counter
• Switches in a mode where the \chapter command uses a different set of styles (= use the same command / heading level, but with adequate derived style)
  • "Appendix" prefix (vs. "Chapter") (default styling, configurable)
  • Alpha numbering (vs. arabic) (default styling, configurable)