title: Neovim: Relative config file(init.vim) location
body: <p>I'm moving from vim to neovim, and my problem is that I need to carry it on portable USB sticks.
  So I need to read config files from relative path. I used <code>$VIM/_vimrc</code> for this purpose on original vim, since <code>$VIM</code> always points to vim files in my USB. 
  But Neovim stores config files in <code>~/.config</code> on Linux or <code>Appdata</code> on Windows, which both points to local computer's directory I was using.
  Is their any relative directory Neovim uses to read config files? I don't want to manually source config file every single time...</p>