vim-plug 自身を Windows でも macOS でも自動インストール ref: http://qiita.com/watagashi/items/cf45ffdd7c21655317a2

.vimrc

let s:vimfiles = expand('<sfile>:p:h')
let s:plugvim = s:vimfiles . '/autoload/plug.vim'
if empty(glob(s:plugvim))
    execute "silent !curl -fLo " . s:plugvim . " --create-dirs "
                \ . "https://raw.githubusercontent.com/junegunn/vim-plug/master/plug.vim"
    autocmd VimEnter * PlugInstall --sync | source $MYVIMRC
endif

if !empty(glob(s:plugvim))
    call plug#begin(s:vimfiles . '/plugged')

    ” インストールしたいプラグインを列挙 (以下は一例)
    Plug 'twitvim/twitvim'
    Plug 'vim-airline/vim-airline'
    Plug 'vim-airline/vim-airline-themes'
    " ...

    call plug#end()
endif