Add support for copy/paste registers

This adds support for vim-style copy/paste registers which allows
cutting and copying multiple items without having to overwrite the
copy/paste buffer. Registers can be specified using the quote key ('"').
To access a register, type '"x' before a command where "x" is the name
of a register. If you want to copy the currently selected item into
register 1, type '"1c'.

Valid registers are 0-9, a-z, "-" and "_". Note that the latter is the
so-called black hole register, which works similar to the black hole
register in vim.

The register prefix key is currently hardcoded and cannot be configured.

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

src/utils.c

@@ -97,12 +97,15 @@ void exit_calcurse(int status)
 
 void free_user_data(void)
 {
+  unsigned i;
+
   day_free_list();
   event_llist_free();
   apoint_llist_free();
   recur_apoint_llist_free();
   recur_event_llist_free();
-  interact_day_item_cut_free();
+  for (i = 0; i <= 37; i++)
+    interact_day_item_cut_free(i);
   todo_free_list();
   notify_free_app();
 }