command: add append-play loadfile mode

"loadfile filename append-play" will now always append the file to the
playlist, and if nothing is playing yet, start playback. I don't want to
change the semantics of "append" mode, so a new mode is needed.

Probably fixes issue #950.
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.
Release 0.4.2