extra-head-content: introduce another option for meta tags

This is to support things like go-import meta tags, which are on a
per-repo basis.

Signed-off-by: Jason A. Donenfeld <Jason@zx2c4.com>

cgit.h

@@ -81,6 +81,7 @@ struct cgit_repo {
 	char *name;
 	char *path;
 	char *desc;
+	char *extra_head_content;
 	char *owner;
 	char *homepage;
 	char *defbranch;