Add the concept of an Asciicast session

[LegNeato]

Add a higher-level concept of an Asciicast session. This would keep track of timing and such. API might look something like:

let mut session = asciicast::Session::new(asciicast::v2);
session.set_height(42);
session.set_width(42);
session.add_event(asciicast::EventType::Input, String::new("this is an event");
session.add_event(asciicast::EventType::Output, String::new("whatever");
let completed = session.end();

It might be cool to have the session provide a stream as well.