Show warning when Oh My Zsh themes are enabled (#426)
Show warning when Oh My Zsh themes are enabled (#426)

The logic here is that if we unset ZSH_THEME during Pure init, we guard
against this scenario:

```
ZSH_THEME=sometheme
prompt pure
source $ZSH/oh-my-zsh.zsh
```

Also, by unsetting ZSH_THEME we can detect the following scenario:

```
prompt pure
ZSH_THEME=othertheme
source $ZSH/oh-my-zsh.zsh
```

And in this case, we show a warning, because there's no telling what
that OMZ theme might have done (precmd hooks, etc).
joshuarubin/dotfiles,sindresorhus/pure,joshuarubin/dotfiles,kgrz/pure,joshuarubin/dotfiles,forivall/pure,jaredru/zsh-pure,joshuarubin/dotfiles