.functions: Improve `targz`

Idea: Would be cool if `targz foo` would create `foo.tar`, then check its file size, and then use it to decide wether to use `zopfli` or `gzip`. Anything over, say, 50 MB, should probably use gzip. Zopfli is great for small files, but quickly becomes too slow as file size increases.

@alrra suggested using fewer Zopfli iterations (`15` is the default) but I’m not sure about that, as it would minimize the benefit of using Zopfli.

@einars suggested using [pigz](http://zlib.net/pigz/) (`brew install pigz`), the fast gzip-compatible multithreaded compressor™. Perhaps another check before falling back to `gzip`?