Support zsh users who do not use oh-my-zsh?

[wtao137]

It turns out the install script assumes a ZSH_CUSTOM env variable to put the completion plugin, which, if I'm correct, normally would come with an oh-my-zsh installation.

However, vanilla zsh can already load the completion file for use. So it's it possible to tweak the installation script a bit so it can also work even if it doesn't detect a ZSH_CUSTOM variable?