Add bundle_dir option

I have add the feature that setting a bundle directory of any one of the plugins, such as:

```VimL
Plugin 'joonty/vim-sauce', {'bundle_dir' : '~/.vim/custom_bundle/'}
```

The follow option can installed `vim-sause` at `~/.vim/custom_bundle` instead of `~/.vim/bundle/vundle/`, which is default runtimepath directory.

Tested, it can be used as another command, such as `PluginInstall` or `Plugin` etc.

The option `bundle_dir` has the following usage scenarios:

- We need place a plugin to a specially directory in order to be packaged;
- We need a same location of vundle directory, which is synchronous with my team by linking dir:

```bash
    .
    |-- bundle 
    `-- team_bundle -> /var/share/common_bundle
```

The feature also solve https://github.com/gmarik/Vundle.vim/issues/125 https://github.com/gmarik/Vundle.vim/issues/350 issues.