Add head-include configuration option.

This patch adds an option to the configuration file, "head-include",
which works just like "header" or "footer", except the content is put
into the HTML's <head> tag.
diff --git a/cgit.h b/cgit.h
index 5f7af51..aed826a 100644
--- a/cgit.h
+++ b/cgit.h
@@ -136,6 +136,7 @@
 	char *css;
 	char *favicon;
 	char *footer;
+	char *head_include;
 	char *header;
 	char *index_header;
 	char *index_info;