Added command completion (ko-build#49)

* Enable command completion (ko-build#27)

* Added documentation (ko-build#27)

README.md

@@ -397,6 +397,23 @@ kubectl logs kodata
 2018/07/19 23:38:29 Hello there
 ```
 
+## Enable Autocompletion
+
+To generate an bash completion script, you can run:
+```
+ko completion
+```
+
+To use the completion script, you can copy the script in your bash_completion directory (e.g. /usr/local/etc/bash_completion.d/):
+```
+ko completion > /usr/local/etc/bash_completion.d/ko
+``` 
+ or source it in your shell by running:
+```
+source <(ko completion)
+```
+
+
 ## Relevance to Release Management
 
 `ko` is also useful for helping manage releases. For example, if your project

pkg/commands/commands.go

@@ -29,4 +29,5 @@ func AddKubeCommands(topLevel *cobra.Command) {
 	addResolve(topLevel)
 	addPublish(topLevel)
 	addRun(topLevel)
+	addCompletion(topLevel)
 }

pkg/commands/completion.go

@@ -0,0 +1,44 @@
+// Copyright 2018 Google LLC All Rights Reserved.
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//    http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+package commands
+
+import (
+	"os"
+
+	"github.com/spf13/cobra"
+)
+
+type CompletionFlags struct {
+	Zsh bool
+}
+
+func addCompletion(topLevel *cobra.Command) {
+	var completionFlags CompletionFlags
+
+	completionCmd := &cobra.Command{
+		Use:   "completion",
+		Short: "Output shell completion code (default Bash)",
+		Run: func(cmd *cobra.Command, args []string) {
+			if completionFlags.Zsh {
+				cmd.Root().GenZshCompletion(os.Stdout)
+			} else {
+				cmd.Root().GenBashCompletion(os.Stdout)
+			}
+		},
+	}
+
+	completionCmd.Flags().BoolVar(&completionFlags.Zsh, "zsh", false, "Generates completion code for Zsh shell.")
+	topLevel.AddCommand(completionCmd)
+}