feat(body): put Stream impl for Body behind stream feature

BREAKING CHANGE: Using a `Body` as a `Stream`, and constructing one via
  `Body::wrap_stream`, require enabling the unstable `stream` feature.

src/lib.rs

@@ -18,6 +18,15 @@
 //!
 //! If looking for just a convenient HTTP client, consider the
 //! [reqwest](https://crates.io/crates/reqwest) crate.
+//!
+//! # Optional Features
+//!
+//! The following optional features are available:
+//!
+//! - `runtime` (*enabled by default*): Enables convenient integration with 
+//!   `tokio`, providing connectors and acceptors for TCP, and a default
+//!   executor.
+//! - `stream` (*unstable*): Provides `futures::Stream` capabilities.
 
 #[doc(hidden)] pub use http;
 #[macro_use] extern crate log;