Add API function idea.

TODO

diff --git a/TODO b/TODO
index 623fa2e..f369be6 100644
--- a/TODO
+++ b/TODO
@@ -17,6 +17,7 @@
 
 Features
 ========
+! function to copy file from one archive to another
 ! Document API changes
 ! Compare headers from last release and next for API changes
 * do not compress if storing is smaller