feature req: handle #![doc]

[joshka]

#![doc] and various versions gated with feature flags etc are useful for:

• including examples from a file into the lib docs using include_str! / include-doc)
• documenting features in the cargo.toml using document_features

E.g.:

//! # Example
//! ```rust
#![doc = include_str!("../examples/hello_world.rs")]
//! ```
//! # Features
#![cfg_attr(feature = "document-features", doc = document_features::document_features!())]

Currently these render as empty in the readme.

[orium]

Once #177 is done this should be relatively easy to do.

[joshka]

Noted - thanks!