add zsh completion

completions/_rbenv.zsh

@@ -0,0 +1,20 @@
+#compdef _rbenv rbenv
+
+function _rbenv_commands() {
+  cmds_str="$(rbenv commands)"
+  cmds=("${(ps:\n:)cmds_str}")
+  _describe '_rbenv_commands' cmds
+}
+
+_rbenv_versions() {
+  versions_str="$(rbenv versions --bare)"
+  versions=(system "${(ps:\n:)versions_str}")
+  _describe '_rbenv_versions' versions
+}
+
+_rbenv() {
+  case "$words[2]" in
+    set-local | set-default | prefix ) _rbenv_versions ;;
+    * ) _rbenv_commands ;;
+  esac
+}