Add option, or default, to zcompile .zcompdump

[segevfiner]

This provides a performance improvement on subsequent Zsh startup. There are even tricks to compile it in the background so it doesn't even effect performance when you do run it, although zcompile shouldn't really take long AGAIK from what it actually does.

[belak]

That's a cool idea. We already have a minor trick to only build the zcompdump only once per day, and if there's an easy way to integrate that with zcompile, I'd happily accept a pull request.