add support for snapshot tarballs

- reworked cgit_print_snapshot to use a list of supported archivers and pick
	one for the suffix supplied
- moved printing of snaphot links into ui-snapshot and make it iterate through
	the said list

cgit.c

@@ -68,7 +68,7 @@ static void cgit_print_repo_page(struct cacheitem *item)
 	setenv("GIT_DIR", cgit_repo->path, 1);
 
 	if ((cgit_cmd == CMD_SNAPSHOT) && cgit_repo->snapshots) {
-		cgit_print_snapshot(item, cgit_query_sha1, "zip",
+		cgit_print_snapshot(item, cgit_query_sha1,
 				    cgit_repo->url, cgit_query_name);
 		return;
 	}