Ignore #[cfg_attr(rustfmt, rustfmt_skip)] functions

This adds to #[rustfmt_skip] but is usable on stable Rust!

src/utils.rs

@@ -75,6 +75,9 @@ pub fn last_line_width(s: &str) -> usize {
 fn is_skip(meta_item: &MetaItem) -> bool {
     match meta_item.node {
         MetaItem_::MetaWord(ref s) => *s == SKIP_ANNOTATION,
+        MetaItem_::MetaList(ref s, ref l) => {
+            *s == "cfg_attr" && l.len() == 2 && is_skip(&l[1])
+        }
         _ => false,
     }
 }