Add 'Unplug' command

I add the command in order to invalid the previous plugin.

For this scenario, I have a vimrc call `vimrc.bundles` which is not allowed to changing:

```VimL
Plugin 'foo'
Plugin 'bar'
```

In my .vimrc, I have to source the `vimrc.bundles` whereas the 'bar' isn't suit for my env, so I can use `Unplug`:

> ~/.vimrc:

```VimL
source vimrc.bundles

" Invalid 'foo'
Unplug 'foo'
" Use Vundle as well
Pulgin 'Align'
" ...
```

It may be suitable for https://github.com/gmarik/Vundle.vim/pull/422#issuecomment-39438671