Reverted the behavior change with --title-prefix.

It now implies `--standalone` again, as before.

MANUAL.txt

@@ -1032,8 +1032,7 @@ header when requesting a document from a URL:
 
 :   Specify *STRING* as a prefix at the beginning of the title
     that appears in the HTML header (but not in the title as it
-    appears at the beginning of the HTML body).  This option
-    should normally be used with `--standalone`.
+    appears at the beginning of the HTML body).  Implies `--standalone`.
 
 `-c` *URL*, `--css=`*URL*
 

src/Text/Pandoc/App/CommandLineOptions.hs

@@ -584,7 +584,8 @@ options =
                   (\arg opt ->
                     return opt {
                        optVariables =
-                         setVariable "title-prefix" arg $ optVariables opt })
+                         setVariable "title-prefix" arg $ optVariables opt,
+                       optStandalone = True })
                   "STRING")
                  "" -- "String to prefix to HTML window title"
 

src/Text/Pandoc/App/Opt.hs

@@ -332,7 +332,8 @@ doOpt (k',v) = do
     "file-scope" ->
       parseYAML v >>= \x -> return (\o -> o{ optFileScope = x })
     "title-prefix" ->
-      parseYAML v >>= \x -> return (\o -> o{ optTitlePrefix = x })
+      parseYAML v >>= \x -> return (\o -> o{ optTitlePrefix = x,
+                                             optStandalone = True })
     "css" ->
       (parseYAML v >>= \x -> return (\o -> o{ optCss = optCss o <>
                                                  map unpack x }))