add brew-cask plugin

plugins/brew-cask/brew-cask.plugin.zsh

+# Autocompletion for homebrew-cask.
+#
+# This script intercepts calls to the brew plugin and adds autocompletion
+# for the cask subcommand.
+#
+# Author: https://github.com/pstadler
+
+compdef _brew-cask brew
+
+_brew-cask()
+{
+	local curcontext="$curcontext" state line
+	typeset -A opt_args
+
+	_arguments -C \
+		':command:->command' \
+		':subcmd:->subcmd' \
+		'*::options:->options'
+
+	case $state in
+		(command)
+			__call_original_brew
+			cask_commands=(
+				'cask:manage casks'
+			)
+			_describe -t commands 'brew cask command' cask_commands ;;
+
+		(subcmd)
+			case "$line[1]" in
+				cask)
+					if (( CURRENT == 3 )); then
+						local -a subcommands
+						subcommands=(
+							"alfred:used to modify Alfred's scope to include the Caskroom"
+							'audit:verifies installability of casks'
+							'checklinks:checks for bad cask links'
+							'create:creates a cask of the given name and opens it in an editor'
+							'edit:edits the cask of the given name'
+							'home:opens the homepage of the cask of the given name'
+							'info:displays information about the cask of the given name'
+							'install:installs the cask of the given name'
+							'list:with no args, lists installed casks; given installed casks, lists installed files'
+							'search:searches all known casks'
+							'uninstall:uninstalls the cask of the given name'
+						)
+						_describe -t commands "brew cask subcommand" subcommands
+					fi ;;
+
+				*)
+					__call_original_brew ;;
+			esac ;;
+
+		(options)
+			local -a casks installed_casks
+			local expl
+			case "$line[2]" in
+				list|uninstall)
+					__brew_installed_casks
+					_wanted installed_casks expl 'installed casks' compadd -a installed_casks ;;
+				audit|edit|home|info|install)
+					__brew_all_casks
+					_wanted casks expl 'all casks' compadd -a casks ;;
+			esac ;;
+	esac
+}
+
+__brew_all_casks() {
+	casks=(`brew cask search`)
+}
+
+__brew_installed_casks() {
+	installed_casks=(`brew cask list`)
+}
+
+__call_original_brew()
+{
+	local ret=1
+	_call_function ret _brew
+	compdef _brew-cask brew
+}