Let metaflac output to a different file

[kalrish]

TL;DR: metaflac should support outputting to a different file, like vorbiscomment(1).

Background: I have created a music compilation system which works kinda like a software build system. When possible, it encodes and tags in different steps so that a mere tag change doesn't result in the reencoding of the entire library. Unfortunately, metaflac doesn't support outputting to a different file (or to stdout), so I have to copy the untagged files manually.

Currently:

flac -o song-untagged.flac song.wav
cp -- song-untagged.flac song.flac
metaflac --set-tag=TITLE=Song song.flac

Desired:

flac -o song-untagged.flac song.wav
metaflac --set-tag=TITLE=Song -o song.flac song-untagged.flac

Since metaflac is designed to operate on multiple files simultaneously and we don't want to break the interface, I think the easiest would be to add a -o option. If it is specified, only one input file may be passed; any more should cause an error.