Teach cgit howto include an external file on index page.

The new parameter index-header can be used to name an external file
which will be included verbatim at the top of the index page.

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

cgit.h

@@ -75,6 +75,7 @@ extern struct repoinfo *cgit_repo;
 extern char *cgit_root_title;
 extern char *cgit_css;
 extern char *cgit_logo;
+extern char *cgit_index_header;
 extern char *cgit_logo_link;
 extern char *cgit_module_link;
 extern char *cgit_virtual_root;