Annotations

[qryxip]

This is a feature request. (I'm also ready to create a PR)

Enable adding attributes to items like cbindgen's annotation.

I come up with three ways to write annotations:

1. Respect cbindgen's annotations

   /// cbindgen:ignore
   pub Item(rust_impl::Item);
   

2. /// csbindgen:annotation

   /// cbindgen:ignore
   /// csbindgen:ignore
   pub Item(rust_impl::Item);
   

3. #[cfg_attr(any(), csbindgen::ignore)]

   /// cbindgen:ignore
   #[cfg_attr(any(), csbindgen::ignore)] // like `#[cfg_attr(any(), rustfmt::skip)]`
   pub Item(rust_impl::Item);