Support for `#[error(forward)]`

[mamekoro]

Could you consider adding support for #[error(forward)] which provide the same functionality as #[error(transparent)] in the thiserror crate? (Use cases are explained there.)

In short, this would make fn source() return self.source.as_dyn_error().source() instead of Some(self.source.as_dyn_error()).

I can use thiserror, but derive_more is a crate I use everywhere, and it would be nice to have this functionality without depending on thiserror.

[mamekoro]

Oops. It seems there's already a PR for this (#293). I was only searching for issues.