Add --prefix-directives and --no-prefix-directives options to ikiwiki

This avoids the need to set prefix_directives with --set.

doc/usage.mdwn

@@ -277,6 +277,11 @@ configuration options of their own.
 
   Toggle creating output files named page/index.html (default) instead of page.html.
 
+* --prefix-directives, --no-prefix-directives
+
+  Toggle new '!'-prefixed syntax for preprocessor directives.  ikiwiki currently
+  defaults to --no-prefix-directives.
+
 * --w3mmode, --no-w3mmode
 
   Enable [[w3mmode]], which allows w3m to use ikiwiki as a local CGI script,

ikiwiki.in

@@ -29,6 +29,7 @@ sub getconfig () { #{{{
 			"render=s" => \$config{render},
 			"wrappers!" => \$config{wrappers},
 			"usedirs!" => \$config{usedirs},
+			"prefix-directives!" => \$config{prefix_directives},
 			"getctime" => \$config{getctime},
 			"numbacklinks=i" => \$config{numbacklinks},
 			"rcs=s" => \$config{rcs},