snapshot: support special value 'all' to enable all formats

Signed-off-by: Christian Hesse <mail@eworm.de>
Reviewed-by: John Keeping <john@keeping.me.uk>
diff --git a/shared.c b/shared.c
index 21ac8f4..0a11e68 100644
--- a/shared.c
+++ b/shared.c
@@ -390,6 +390,9 @@
 	if (atoi(str))
 		return 1;
 
+	if (strcmp(str, "all") == 0)
+		return INT_MAX;
+
 	string_list_split(&tokens, str, ' ', -1);
 	string_list_remove_empty_items(&tokens, 0);