Add Shell Completion script generation

`dnsutils shell-completion <shell>` will generate a shell completion
script for the specified shell (bash or zsh). If no shell is specified,
the script will be generated for the current shell, using `$SHELL`.

commands/completion-scripts/completion.zsh.gotmpl

@@ -0,0 +1,20 @@
+#compdef "{{.App.Name}}"
+
+_dnscontrol() {
+  local -a opts
+  local cur
+  cur=${words[-1]}
+  if [[ "$cur" == "-"* ]]; then
+    opts=("${(@f)$(${words[@]:0:#words[@]-1} ${cur} --generate-bash-completion)}")
+  else
+    opts=("${(@f)$(${words[@]:0:#words[@]-1} --generate-bash-completion)}")
+  fi
+
+  if [[ "${opts[1]}" != "" ]]; then
+    _describe 'values' opts
+  else
+    _files
+  fi
+}
+
+compdef "_dnscontrol" "{{.App.Name}}"