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/`, 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;
```VimL
Plugin 'sickill/vim-monokai', {'bundle_dir' : '~/vim_theme/'}
```

- We need a same location of vundle directory, which is synced my team by linking dir:

```bash
/home/leiming/.vim 
|-- bundle
|   |-- vundle/
|   |-- phpctags.vim/
|   |-- DoxygenToolkit.vim/
|   `-- tagbar/
|-- team_bundle -> /var/share/common_bundle # Symbolic links is optional
|   |-- ctrlp.vim/
|   |-- vim-nerdtree-tabs/
|   `-- nerdtree/
|-- plugin
|-- after
...
# You should ensure that the directory like
#  `/var/share/common_bundle` has permission to read and write.
```

So we can simply set the installed directory like this:

```VimL
" Your local plugins, which are installed in `vundle#rc(path)`: ~/.vim/bundle/
Plugin 'majutsushi/tagbar'
Plugin 'vim-php/phpctags'
" ...

let team_bundle_dir=expand('~/.vim/common_bundle/') " As your favourite

" Team plugin
Plugin 'kien/ctrlp.vim',                      {'bundle_dir': team_bundle_dir}
Plugin 'scrooloose/nerdtree',                 {'bundle_dir': team_bundle_dir}
Plugin 'jistr/vim-nerdtree-tabs',             {'bundle_dir': team_bundle_dir}
"  ...

```

The feature also solve https://github.com/gmarik/Vundle.vim/issues/125 https://github.com/gmarik/Vundle.vim/issues/350 issues.