Merge pull request #1999 from akatov/carton

cask completion plugin

plugins/cask/cask.plugin.zsh

@@ -0,0 +1,5 @@
+if which cask &> /dev/null; then
+  source $(dirname $(which cask))/../etc/cask_completion.zsh
+else
+  print "zsh cask plugin: cask not found"
+fi