Make automatic installation script in wiki tips compatible to windows

Current [automatic installation script in wiki](https://github.com/junegunn/vim-plug/wiki/tips#automatic-installation) is not compatible with Windows, mostly due to these two problems:
1. `autoload` should be in `~/vimfiles` instead of `~/.vim` on Windows.
2. The cURL comes with Windows (namely `C:/Windows/System32/curl.exe`) and cmd.exe/PowerShell will not expand `~` with `!` invocation.

I wrote a compatible version, tested to solve these two problems, making it compatible with Windows:
```vim
let data_dir = has('nvim') ? stdpath('data') . '/site' : has('win32') ? '~/vimfiles' : '~/.vim'
if empty(glob(data_dir . '/autoload/plug.vim'))
silent execute '!curl -fLo '.fnamemodify(data_dir.'/autoload/plug.vim', ':p:S').' --create-dirs https://raw.githubusercontent.com/junegunn/vim-plug/master/plug.vim'
autocmd VimEnter * PlugInstall --sync | source $MYVIMRC
endif
```
Only few words changed.

If we get this updated to wiki, we can benefit the users.