plenv-init : add --no-completion option 

I add '--no-completion' option to plenv-init.
plenv-init force to define _plenv on zsh.
But I want to use my _plnev function. Then, I use plenv like bellow now

```zsh
  eval "$(plenv init -)"
  unfunction _plenv
  autoload -U _plenv
```

If you merge, I can write bellow

```zsh
  eval "$(plevn init - --no-completion)"
```