make zsh completion autoloadable

This is the standard way to add completions to zsh. For the completions to be autoloaded, the file _cask must be in the $fpath of zsh, typically /usr/share/zsh/site-functions.

To add the zsh completions manually, the cask/etc directory should be added to $fpath in .zshrc, instead of sourcing the file, like it was documented in the README about a year ago.

From what I understand, this would also make it possible to autoload the autocompletion when installing from homebrew.