Add key binding for pipe-item command

This removes the need of reading the whole data file and find matching
entries if we want to parse appointments in external programs.

Signed-off-by: Lukas Fleischer <calcurse@cryptocrack.de>

src/calcurse.h

@@ -401,6 +401,7 @@ enum key {
   KEY_DEL_ITEM,
   KEY_EDIT_ITEM,
   KEY_VIEW_ITEM,
+  KEY_PIPE_ITEM,
   KEY_FLAG_ITEM,
   KEY_REPEAT_ITEM,
   KEY_EDIT_NOTE,