Enable shell completion (#1559)

* feat: enable EnableBashCompletion to get --generate-bash-completion command line flag

* feat: add scripts for bash and zsh

* feat: document shell completion

autocomplete/helmfile_bash_autocomplete

@@ -0,0 +1,18 @@
+#! /bin/bash
+
+_helmfile_bash_autocomplete() {
+  if [[ "${COMP_WORDS[0]}" != "source" ]]; then
+    local cur opts base
+    COMPREPLY=()
+    cur="${COMP_WORDS[COMP_CWORD]}"
+    if [[ "$cur" == "-"* ]]; then
+      opts=$( ${COMP_WORDS[@]:0:$COMP_CWORD} ${cur} --generate-bash-completion )
+    else
+      opts=$( ${COMP_WORDS[@]:0:$COMP_CWORD} --generate-bash-completion )
+    fi
+    COMPREPLY=( $(compgen -W "${opts}" -- ${cur}) )
+    return 0
+  fi
+}
+
+complete -o bashdefault -o default -o nospace -F _helmfile_bash_autocomplete helmfile

autocomplete/helmfile_zsh_autocomplete

@@ -0,0 +1,23 @@
+#compdef helmfile
+
+_helmfile_zsh_autocomplete() {
+
+  local -a opts
+  local cur
+  cur=${words[-1]}
+  if [[ "$cur" == "-"* ]]; then
+    opts=("${(@f)$(_CLI_ZSH_AUTOCOMPLETE_HACK=1 ${words[@]:0:#words[@]-1} ${cur} --generate-bash-completion)}")
+  else
+    opts=("${(@f)$(_CLI_ZSH_AUTOCOMPLETE_HACK=1 ${words[@]:0:#words[@]-1} --generate-bash-completion)}")
+  fi
+
+  if [[ "${opts[1]}" != "" ]]; then
+    _describe 'values' opts
+  else
+    _files
+  fi
+
+  return
+}
+
+compdef _helmfile_zsh_autocomplete helmfile