Add variable interpolation TODO

Signed-off-by: martin f. krafft <madduck@madduck.net>
diff --git a/TODO b/TODO
index 050608c..d8e97cf 100644
--- a/TODO
+++ b/TODO
@@ -8,3 +8,4 @@
 - Improve the adapters, either by just letting them build their own
   OptionParsers, or through setuptools entrypoints (probably best…). The
   reclass API would need a bit of generalisation for that to happen…
+- Variable interpolation