Add zsh completion script (finally)

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

Formula/kubectx.rb

@@ -0,0 +1,18 @@
+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/v0.2.0.tar.gz"
+  sha256 "9fb6557416e4be3ef7e9701527f89fa73a1a0545a3cafbe6bed7527061c6cfb7"
+
+  bottle :unneeded
+
+  def install
+    bin.install "kubectx"
+    bash_completion.install "completion/kubectx.bash" => "kubectx"
+    zsh_completion.install "completion/kubectx.zsh" => "_kubectx"
+  end
+
+  test do
+    system "which", "kubectx"
+  end
+end

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