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