Add zsh completion script (finally)

Signed-off-by: Ahmet Alp Balkan <ahmetb@google.com>

Formula/kubectx.rb

@@ -1,21 +1,15 @@
 class Kubectx < Formula
   desc "Tool that can switch between kubectl contexts easily and create aliases"
   homepage "https://github.com/ahmetb/kubectx"
-  url "https://github.com/ahmetb/kubectx/archive/v0.1.tar.gz"
-  sha256 "841817f928af25061b1b6794400394c3e6e807e8a1c48c179f1fd8bdd553ca79"
+  url "https://github.com/ahmetb/kubectx/archive/v0.2.0.tar.gz"
+  sha256 "9fb6557416e4be3ef7e9701527f89fa73a1a0545a3cafbe6bed7527061c6cfb7"
 
   bottle :unneeded
 
   def install
     bin.install "kubectx"
     bash_completion.install "completion/kubectx.bash" => "kubectx"
-  end
-
-  def caveats; <<-EOS.undent
-    To install zsh completion, add this to your .zshrc:
-
-      [ -f /usr/local/etc/bash_completion.d/kubectx ] && source /usr/local/etc/bash_completion.d/kubectx
-    EOS
+    zsh_completion.install "completion/kubectx.zsh" => "_kubectx"
   end
 
   test do

completion/kubectx.zsh

@@ -0,0 +1,12 @@
+#compdef kubectx
+
+KUBECTX="${HOME}/.kube/kubectx"
+PREV=""
+if [ -f "$KUBECTX" ]; then
+    # show '-' only if there's a saved previous context
+    PREV=$(cat "${KUBECTX}")
+    _arguments "1: :((-\:Back\ to\ ${PREV} \
+        $(kubectl config get-contexts | awk '{print $2}' | tail -n +2)))"
+else
+    _arguments "1: :($(kubectl config get-contexts | awk '{print $2}' | tail -n +2))"
+fi