add zsh-autopair

This plugin automatically adds a pair to specified characters like quotes, brackets, etc; removes a huge annoyance.

Clone the plugin to Oh My Zsh's custom plugins directory

```shell
git clone https://github.com/hlissner/zsh-autopair \
  ${ZSH_CUSTOM:-~/.oh-my-zsh/custom}/plugins/zsh-autopair
```

Then add it to your plugins array in `~/.zshrc`

```shell
plugins=(... zsh-autopair)
```
