[BUGFIX] Support $ZSH_CUSTOM instead of using hard coded paths

Currently the path to the custom folder is hard coded. But according to https://github.com/robbyrussell/oh-my-zsh/wiki/Customization#using-another-customization-directory we should use `$ZSH_CUSTOM`

> Initially $ZSH_CUSTOM points to oh-my-zsh's custom directory.
> [...]
> If you don't want to use the built-in custom directory itself, just change the path of $ZSH_CUSTOM inside your .zshrc to a directory of your own liking. Everything will be fine as long as you adhere to the conventional file hierarchy.