buf: Add BufStreamExt trait and add a core feature (#897)

This change adds an extension trait to `BufStream` and puts the core
trait behind a feature flag for optional use.

This mainly adds the additional functions in an extension trait to
allow the user to select if they want just the core trait or the fully
featured version. Now the user can add the core feature to _not_
include the extension trait. By deafult, this feature is disabled.

tokio-buf/README.md

@@ -0,0 +1,35 @@
+# tokio-buf
+
+Asynchronous stream of byte buffers
+
+[Documenation](https://docs.rs/tokio-buf)
+
+## Usage
+
+First, add this to your `Cargo.toml`:
+
+```toml
+[dependencies]
+tokio-buf = "0.1"
+```
+
+Next, add this to your crate:
+
+```rust
+extern crate tokio_buf;
+```
+
+You can find extensive documentation and examples about how to use this crate
+online at [https://tokio.rs](https://tokio.rs). The [API
+documentation](https://docs.rs/tokio-buf) is also a great place to get started
+for the nitty-gritty.
+
+## License
+
+This project is licensed under the [MIT license](LICENSE).
+
+### Contribution
+
+Unless you explicitly state otherwise, any contribution intentionally submitted
+for inclusion in Tokio by you, shall be licensed as MIT, without any additional
+terms or conditions.