Fix detecting indentation in some CSS

This is done by ignoring lines that don't have any
alphanumeric characters, e.g. following would be detected
as 4-spaces indentation instead of 2-spaces

.android {
    height: 404px; width: 334px;
    margin: 100px auto;
  }

https://github.com/tpope/vim-sleuth/issues/22

plugin/polyglot.vim

@@ -36,7 +36,7 @@ function! s:guess(lines) abort
   let spaces_minus_tabs = 0
 
   for line in a:lines
-    if !len(line) || line =~# '^\s*$'
+    if !len(line) || line =~# '^\W*$'
       continue
     endif