Add reorder_attributes config to sort doc attributes before other attributes

[dtolnay]

This was discussed in rust-lang/style-team#72 with the conclusion that the style guide recommends doc comments before attributes, with an option in rustfmt to do the reordering that is off by default.

I did not find an existing option that does this, nor an issue to track, so here we go. :)

Expected diff:

- #[cfg(cfg)]
  /// doc
+ #[cfg(cfg)]
  struct S;

[ctaggart]

I think this issue and #3744 are the same. Even though this came first, #3744 has more active discussion. I'd recommend closing this @dtolnay.

[ytmimi]

Going to close this in favor of #3744