command: append entries to the end of the playlist with loadlist append

Currently entries are added after the current playlist element. This is kinda confusing, more so given that "loadfile append" appends at the end of the playlist.

Not sure if it'd be better to simply inline the while loop, since the new function is used only once.

Anyway, I've recently started using "loadlist append" quite frequently with grooved, and the "append after the end" is the behavior I expected but was quite confused when this didn't happen. Maybe the old behavior could be readded with a new option or something if needed.