toggle relative number based on autocmd events

autocommands.vim

@@ -50,4 +50,10 @@ augroup auto_read
     autocmd FileChangedShellPost * echohl WarningMsg
                 \ | echo "File changed on disk. Buffer reloaded!" | echohl None
 augroup END
+
+augroup numbertoggle
+  autocmd!
+  autocmd BufEnter,FocusGained,InsertLeave,WinEnter * if &nu | set rnu   | endif
+  autocmd BufLeave,FocusLost,InsertEnter,WinLeave   * if &nu | set nornu | endif
+augroup END
 "}