Extract config file handlers into a separate file

We used custom_load_conf() to load the configuration file and
io_save_conf() to save configuration. Move these functions, including
all helpers, to a central location.

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

src/calcurse.c

@@ -158,7 +158,7 @@ main (int argc, char **argv)
    * configuration (the display is then updated), and then
    * the todo list, appointments and events.
    */
-  custom_load_conf ();
+  config_load ();
   wins_erase_status_bar ();
   io_load_keys (conf.pager);
   io_load_todo ();