Changed -V so that you can specify a key without a value.

Such keys get the value `true`.

README

@@ -256,12 +256,13 @@ General writer options
     template appropriate for the output format will be used (see
     `-D/--print-default-template`).
 
-`-V` *KEY=VAL*, `--variable=`*KEY:VAL*
+`-V` *KEY[=VAL]*, `--variable=`*KEY[:VAL]*
 :   Set the template variable *KEY* to the value *VAL* when rendering the
     document in standalone mode. This is generally only useful when the
     `--template` option is used to specify a custom template, since
     pandoc automatically sets the variables used in the default
-    templates.
+    templates.  If no *VAL* is specified, the key will be given the
+    value `true`.
 
 `-D` *FORMAT*, `--print-default-template=`*FORMAT*
 :   Print the default template for an output *FORMAT*. (See `-t`

src/pandoc.hs

@@ -295,14 +295,13 @@ options =
 
     , Option "V" ["variable"]
                  (ReqArg
-                  (\arg opt ->
-                     case break (`elem` ":=") arg of
-                          (k,_:v) -> do
-                            let newvars = optVariables opt ++ [(k,v)]
-                            return opt{ optVariables = newvars }
-                          _  -> err 17 $
-                            "Could not parse `" ++ arg ++ "' as a key/value pair (k=v or k:v)")
-                  "KEY:VALUE")
+                  (\arg opt -> do
+                     let (key,val) = case break (`elem` ":=") arg of
+                                       (k,_:v) -> (k,v)
+                                       (k,_)   -> (k,"true")
+                     let newvars = optVariables opt ++ [(key,val)]
+                     return opt{ optVariables = newvars })
+                  "KEY[:VALUE]")
                  "" -- "Use custom template"
 
     , Option "D" ["print-default-template"]