Allow --slide-level=0.

When the slide level is set to 0, headings won't be used at all
in splitting the document into slides. Horizontal rules must be
used to separate slides.

Closes #7476.

MANUAL.txt

@@ -1025,11 +1025,13 @@ header when requesting a document from a URL:
 
 :   Specifies that headings with the specified level create
     slides (for `beamer`, `s5`, `slidy`, `slideous`, `dzslides`).  Headings
-    above this level in the hierarchy are used to divide the
-    slide show into sections; headings below this level create
-    subheads within a slide.  The default is to set the slide level based
-    on the contents of the document; see [Structuring the slide
-    show].
+    above this level in the hierarchy are used to divide the slide show
+    into sections; headings below this level create subheads within a slide.
+    Valid values are 0-6.  If a slide level of 0 is specified, slides will
+    not be split automatically on headings, and horizontal rules must be used
+    to indicate slide boundaries.  If a slide level is not specified
+    explicitly, the slide level will be set automatically based on
+    the contents of the document; see [Structuring the slide show].
 
 `--section-divs`
 

src/Text/Pandoc/App/CommandLineOptions.hs

@@ -576,10 +576,10 @@ options =
                  (ReqArg
                   (\arg opt ->
                       case safeStrRead arg of
-                           Just t | t >= 1 && t <= 6 ->
+                           Just t | t >= 0 && t <= 6 ->
                                     return opt { optSlideLevel = Just t }
                            _      -> E.throwIO $ PandocOptionError
-                                    "slide level must be a number between 1 and 6")
+                                    "slide level must be a number between 0 and 6")
                  "NUMBER")
                  "" -- "Force header level for slides"