Detect WSL (Neovim only) (#887)

`has('wsl')` works since Neovim v0.3.0 (neovim/neovim@5d2dd2e)

Fix: #821

Author: janlazo

plug.vim

@@ -1036,7 +1036,7 @@ function! s:update_impl(pull, force, args) abort
   let s:clone_opt = get(g:, 'plug_shallow', 1) ?
         \ '--depth 1' . (s:git_version_requirement(1, 7, 10) ? ' --no-single-branch' : '') : ''
 
-  if has('win32unix')
+  if has('win32unix') || has('wsl')
     let s:clone_opt .= ' -c core.eol=lf -c core.autocrlf=input'
   endif