Remove nobackup option (#86)

Close #85

The `backup` (and `nobackup`) option is global and not local to the
buffer so this change affects all buffers. This is not the intended.

Note that it seems the `backup` option does not have any effect while
suda write the file through `BufWriteCmd`. So this simple change should
not affect the security of the plugin.

autoload/suda.vim

@@ -186,7 +186,7 @@ function! suda#BufReadCmd() abort
           \})
     silent 0delete _
     setlocal buftype=acwrite
-    setlocal nobackup noswapfile noundofile
+    setlocal noswapfile noundofile
     setlocal nomodified
     filetype detect
     redraw | echo echo_message