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>

third_party/kubernetes/pkg/kubectl/cmd/completion.go

@@ -0,0 +1,172 @@
+/*
+The original code is from https://github.com/kubernetes/kubernetes/blob/master/pkg/kubectl/cmd/completion.go -
+
+Copyright 2016 The Kubernetes Authors.
+Modifications Copyright 2018 the Heptio Ark contributors.
+
+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 cmd
+
+import (
+	"bytes"
+	"github.com/spf13/cobra"
+	"io"
+)
+
+func GenZshCompletion(out io.Writer, ark *cobra.Command) {
+
+	zshHead := "#compdef ark\n"
+
+	out.Write([]byte(zshHead))
+
+	zshInitialization := `
+__ark_bash_source() {
+	alias shopt=':'
+	alias _expand=_bash_expand
+	alias _complete=_bash_comp
+	emulate -L sh
+	setopt kshglob noshglob braceexpand
+	source "$@"
+}
+__ark_type() {
+	# -t is not supported by zsh
+	if [ "$1" == "-t" ]; then
+		shift
+		# fake Bash 4 to disable "complete -o nospace". Instead
+		# "compopt +-o nospace" is used in the code to toggle trailing
+		# spaces. We don't support that, but leave trailing spaces on
+		# all the time
+		if [ "$1" = "__ark_compopt" ]; then
+			echo builtin
+			return 0
+		fi
+	fi
+	type "$@"
+}
+__ark_compgen() {
+	local completions w
+	completions=( $(compgen "$@") ) || return $?
+	# filter by given word as prefix
+	while [[ "$1" = -* && "$1" != -- ]]; do
+		shift
+		shift
+	done
+	if [[ "$1" == -- ]]; then
+		shift
+	fi
+	for w in "${completions[@]}"; do
+		if [[ "${w}" = "$1"* ]]; then
+			echo "${w}"
+		fi
+	done
+}
+__ark_compopt() {
+	true # don't do anything. Not supported by bashcompinit in zsh
+}
+__ark_ltrim_colon_completions()
+{
+	if [[ "$1" == *:* && "$COMP_WORDBREAKS" == *:* ]]; then
+		# Remove colon-word prefix from COMPREPLY items
+		local colon_word=${1%${1##*:}}
+		local i=${#COMPREPLY[*]}
+		while [[ $((--i)) -ge 0 ]]; do
+			COMPREPLY[$i]=${COMPREPLY[$i]#"$colon_word"}
+		done
+	fi
+}
+__ark_get_comp_words_by_ref() {
+	cur="${COMP_WORDS[COMP_CWORD]}"
+	prev="${COMP_WORDS[${COMP_CWORD}-1]}"
+	words=("${COMP_WORDS[@]}")
+	cword=("${COMP_CWORD[@]}")
+}
+__ark_filedir() {
+	local RET OLD_IFS w qw
+	__ark_debug "_filedir $@ cur=$cur"
+	if [[ "$1" = \~* ]]; then
+		# somehow does not work. Maybe, zsh does not call this at all
+		eval echo "$1"
+		return 0
+	fi
+	OLD_IFS="$IFS"
+	IFS=$'\n'
+	if [ "$1" = "-d" ]; then
+		shift
+		RET=( $(compgen -d) )
+	else
+		RET=( $(compgen -f) )
+	fi
+	IFS="$OLD_IFS"
+	IFS="," __ark_debug "RET=${RET[@]} len=${#RET[@]}"
+	for w in ${RET[@]}; do
+		if [[ ! "${w}" = "${cur}"* ]]; then
+			continue
+		fi
+		if eval "[[ \"\${w}\" = *.$1 || -d \"\${w}\" ]]"; then
+			qw="$(__ark_quote "${w}")"
+			if [ -d "${w}" ]; then
+				COMPREPLY+=("${qw}/")
+			else
+				COMPREPLY+=("${qw}")
+			fi
+		fi
+	done
+}
+__ark_quote() {
+    if [[ $1 == \'* || $1 == \"* ]]; then
+        # Leave out first character
+        printf %q "${1:1}"
+    else
+    	printf %q "$1"
+    fi
+}
+autoload -U +X bashcompinit && bashcompinit
+# use word boundary patterns for BSD or GNU sed
+LWORD='[[:<:]]'
+RWORD='[[:>:]]'
+if sed --help 2>&1 | grep -q GNU; then
+	LWORD='\<'
+	RWORD='\>'
+fi
+__ark_convert_bash_to_zsh() {
+	sed \
+	-e 's/declare -F/whence -w/' \
+	-e 's/_get_comp_words_by_ref "\$@"/_get_comp_words_by_ref "\$*"/' \
+	-e 's/local \([a-zA-Z0-9_]*\)=/local \1; \1=/' \
+	-e 's/flags+=("\(--.*\)=")/flags+=("\1"); two_word_flags+=("\1")/' \
+	-e 's/must_have_one_flag+=("\(--.*\)=")/must_have_one_flag+=("\1")/' \
+	-e "s/${LWORD}_filedir${RWORD}/__ark_filedir/g" \
+	-e "s/${LWORD}_get_comp_words_by_ref${RWORD}/__ark_get_comp_words_by_ref/g" \
+	-e "s/${LWORD}__ltrim_colon_completions${RWORD}/__ark_ltrim_colon_completions/g" \
+	-e "s/${LWORD}compgen${RWORD}/__ark_compgen/g" \
+	-e "s/${LWORD}compopt${RWORD}/__ark_compopt/g" \
+	-e "s/${LWORD}declare${RWORD}/builtin declare/g" \
+	-e "s/\\\$(type${RWORD}/\$(__ark_type/g" \
+	<<'BASH_COMPLETION_EOF'
+`
+	out.Write([]byte(zshInitialization))
+
+	buf := new(bytes.Buffer)
+	ark.GenBashCompletion(buf)
+	out.Write(buf.Bytes())
+
+	zshTail := `
+BASH_COMPLETION_EOF
+}
+__ark_bash_source <(__ark_convert_bash_to_zsh)
+_complete ark 2>/dev/null
+`
+	out.Write([]byte(zshTail))
+}