Add support for a custom header

Signed-off-by: Lars Hjemli <hjemli@gmail.com>

cgitrc.5.txt

@@ -84,6 +84,10 @@ footer
 	verbatim at the bottom of all pages (i.e. it replaces the standard
 	"generated by..." message. Default value: none.
 
+header
+	The content of the file specified with this option will be included
+	verbatim at the top of all pages. Default value: none.
+
 include
 	Name of a configfile to include before the rest of the current config-
 	file is parsed. Default value: none.