Add TODO item about YAML filename extensions

Signed-off-by: martin f. krafft <madduck@madduck.net>
diff --git a/TODO b/TODO
index d728911..de087ab 100644
--- a/TODO
+++ b/TODO
@@ -4,4 +4,4 @@
 - Salt and Puppet adapters, whoever might want them.
 - Tests for outputters
 - Improve testing of yaml_fs, maybe with more realistic examples
-- 
+- Configurable file extension (.yaml/.yml, or support both)