Allow adding tracks to playlist

[catharsis]

It'd be very helpful to be able to cp a track from, say, a search result directory to a playlist and have that track be added to the playlist.

I'm imagining something like:

mkdir search/Trogdor
cd search/Trogdor
cp ./The\ Bearded\ Bard\ -\ Trogdor\ The\ Burninator.wav ../../playlists/My\ Sweet\ Tracks/

Open questions:

• Semantically, it would perhaps be a bit surprising to copy a link to a track and end up with a new link without requesting explicitly that no dereferencing of symbolic links is desireed: cp tracklink target vs cp -P tracklink target. This could be a usability/semantics tradeoff that would have to be settled.