Add support for including a footer on all pages

The new cgitrc option `footer` can be used to include a html-file which
replaces the standard 'generated by cgit' message at the bottom of each
page.

Suggested-by: Peter Danenberg <pcd@wikitex.org>
Signed-off-by: Lars Hjemli <hjemli@gmail.com>

cgit.h

@@ -125,6 +125,7 @@ struct cgit_config {
 	char *cache_root;
 	char *clone_prefix;
 	char *css;
+	char *footer;
 	char *index_header;
 	char *index_info;
 	char *logo;