Adding a symlink for compatibility with oh-my-zsh

The oh-my-zsh plugin will only include plugins with an extension of `plugin.zsh`. This filename change would be the only change necessary to produce compatibility with oh-my-zsh for `opp.zsh`, but would be understandably confusing for those who do not use oh-my-zsh. The [zsh-syntax-highlighting](https://github.com/zsh-users/zsh-syntax-highlighting) plugin has handled this by including a symlink. This pull request simply does the same thing.