XDG_DATA_HOME respecting powershell script (#1042)

This version supports Powershell 5.1 (shipped in the latest install of Windows 10)

README.md

@@ -68,7 +68,7 @@ curl -fLo ~/.var/app/io.neovim.nvim/data/nvim/site/autoload/plug.vim \
 
 ```powershell
 iwr -useb https://raw.githubusercontent.com/junegunn/vim-plug/master/plug.vim |`
-    ni "$env:LOCALAPPDATA/nvim-data/site/autoload/plug.vim" -Force
+    ni "$(@($env:XDG_DATA_HOME, $env:LOCALAPPDATA)[$null -eq $env:XDG_DATA_HOME])/nvim-data/site/autoload/plug.vim" -Force
 ```
 
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