Add an Iterator on Read that provides both the parsed event and the byte sequence that defines it

Created by: ftilde

This is useful, e.g., for implementing a terminal multiplexer where the raw input should in some cases be passed on to another tty.

Some thoughts on the implementation:

• The function that creates the trait is implemented in a separate extension trait. This is kind of ugly, but (as far as I understand) necessary to ensure backwards compatibility. Although I don't see why the would, users may have implemented TermRead for their own types. My suggestion is to provide the newly added function events_and_raw in a separate trait for following point releases, but merge it into TermRead in a future 2.0 release of Termion.
• The byte sequence is provided as a Vec<u8>. This follows the definition of Event::Unsupported. Other options may include Box<[u8]>, which is smaller on the stack (as far as i know), but less versatile. We could also pull in smallvec and use something like SmallVec4<u8>, but I am not sure if this is worth it. Tell me if you would like me to change the type.