Add bash and zsh completion support

This commit adds support for auto completion for bash and zsh
shells. A new root level command called "completion" has been
introduced, and the user can get the auto completion code by
running `ark completion bash/zsh`.

For bash completion, the built-in GenBashCompletion() from cobra
has been used, but for zsh, the built-in GenZshCompletion() is
known to cause issues. The workaround has been copied from zsh
completion code of kubectl.

Signed-off-by: Shubham <shubham@linux.com>

docs/cli-reference/ark.md

@@ -32,6 +32,7 @@ operations can also be performed as 'ark backup get' and 'ark schedule create'.
 ### SEE ALSO
 * [ark backup](ark_backup.md)	 - Work with backups
 * [ark client](ark_client.md)	 - Ark client related commands
+* [ark completion](ark_completion.md)	 - Output shell completion code for the specified shell (bash or zsh)
 * [ark create](ark_create.md)	 - Create ark resources
 * [ark delete](ark_delete.md)	 - Delete ark resources
 * [ark describe](ark_describe.md)	 - Describe ark resources

docs/cli-reference/ark_completion.md

@@ -0,0 +1,46 @@
+## ark completion
+
+Output shell completion code for the specified shell (bash or zsh)
+
+### Synopsis
+
+
+Generate shell completion code.
+
+Auto completion supports both bash and zsh. Output is to STDOUT.
+
+Load the ark completion code for bash into the current shell -
+source <(ark completion bash)
+
+Load the ark completion code for zsh into the current shell -
+source <(ark completion zsh)
+
+
+```
+ark completion SHELL [flags]
+```
+
+### Options
+
+```
+  -h, --help   help for completion
+```
+
+### Options inherited from parent commands
+
+```
+      --alsologtostderr                  log to standard error as well as files
+      --kubeconfig string                Path to the kubeconfig file to use to talk to the Kubernetes apiserver. If unset, try the environment variable KUBECONFIG, as well as in-cluster configuration
+      --kubecontext string               The context to use to talk to the Kubernetes apiserver. If unset defaults to whatever your current-context is (kubectl config current-context)
+      --log_backtrace_at traceLocation   when logging hits line file:N, emit a stack trace (default :0)
+      --log_dir string                   If non-empty, write log files in this directory
+      --logtostderr                      log to standard error instead of files
+  -n, --namespace string                 The namespace in which Ark should operate (default "heptio-ark")
+      --stderrthreshold severity         logs at or above this threshold go to stderr (default 2)
+  -v, --v Level                          log level for V logs
+      --vmodule moduleSpec               comma-separated list of pattern=N settings for file-filtered logging
+```
+
+### SEE ALSO
+* [ark](ark.md)	 - Back up and restore Kubernetes cluster resources.
+