update save_queue function and add alt command (#21)

- add option to save full or unwatched videos only
- fix when only 1 valid video
- fix error handling
- show messages on save/load

mpv-youtube-queue.conf

@@ -1,4 +1,5 @@
 add_to_queue=ctrl+a
+default_save_method=unwatched
 download_current_video=ctrl+d
 download_selected_video=ctrl+D
 move_cursor_down=ctrl+j
@@ -12,6 +13,7 @@ play_previous_in_queue=ctrl+p
 print_current_video=ctrl+P
 print_queue=ctrl+q
 save_queue=ctrl+s
+save_full_queue=ctrl+S
 remove_from_queue=ctrl+x
 play_selected_video=ctrl+ENTER
 browser=firefox