Merge branch 'apjanke-z-allow-custom'

Closes #4578

plugins/z/z.plugin.zsh

-_load_z() {
-  source $1/z.sh
-}
-
-[[ -f $ZSH_CUSTOM/plugins/z/z.plugin.zsh ]] && _load_z $ZSH_CUSTOM/plugins/z
-[[ -f $ZSH/plugins/z/z.plugin.zsh ]] && _load_z $ZSH/plugins/z
+source "${0:h}/z.sh"