chapter_numbering yaml option: turn numbering off or start at a value

The automatic chapter numbering can now be controlled from the yaml
header. The default TRUE numbers the # chapters from 1 as before.
chapter_numbering: false turns the numbering off -- headings keep
exactly what was typed, including manually typed numbers, and the
overview TOC shows the titles as typed (the numbering script is not
emitted at all). A number starts the enumeration at that value, e.g.
chapter_numbering: 5 for a continuation deck; references, appendix and
thank-you chapters stay excluded and uncounted in every mode.

Change-Id: I69af1414aedd14c9be09650c78b3219bf63c4d1a
diff --git a/man/revealjs_presentation.Rd b/man/revealjs_presentation.Rd
index 2741765..c006418 100644
--- a/man/revealjs_presentation.Rd
+++ b/man/revealjs_presentation.Rd
@@ -8,6 +8,7 @@
   incremental = FALSE,
   center = FALSE,
   slide_level = 2,
+  chapter_numbering = TRUE,
   toc = FALSE,
   toc_depth = 3,
   fig_width = 8,
@@ -49,6 +50,14 @@
 building vertically. It is not recommended that you use deeper nesting of
 section levels with reveal.js.}
 
+\item{chapter_numbering}{\code{TRUE} (the default) numbers the \verb{#} chapter
+headings automatically ("1. Introduction", "2. Methods", ...), skipping
+references, appendix and thank-you chapters. \code{FALSE} turns the numbering
+off -- headings keep exactly what was typed, including any manually
+typed numbers. A number starts the enumeration at that value:
+\code{chapter_numbering = 5} numbers the first chapter "5." (useful for
+continuation decks).}
+
 \item{toc}{\code{TRUE} to include a table of contents in the output (only
 level 1 headers will be included in the table of contents).}
 
@@ -280,7 +289,9 @@
 the overview slide's table of contents uses them as its ordered list's
 enumeration. Decks rendered with \code{slide_level = 1} (one \verb{#} heading per
 slide) are left unnumbered -- there, headings denote slides, not
-chapters.
+chapters. The numbering can be turned off with \code{chapter_numbering: false}
+in the yaml header, or started at another number with e.g.
+\code{chapter_numbering: 5} (useful for continuation decks).
 }
 
 \subsection{Overview slides}{