v0.3.0-beta.2

Changed

• Remove custom termion backends. This is motivated by the fact that
  termion structs are meant to be combined/wrapped to provide additional
  functionalities to the terminal (e.g AlternateScreen, Mouse support, ...).
  Thus providing exclusive types do not make a lot of sense and give a false
  hint that additional features cannot be used together. The recommended
  approach is now to create your own version of stdout:

let stdout = io::stdout().into_raw_mode()?;
let stdout = MouseTerminal::from(stdout);
let stdout = AlternateScreen::from(stdout);

and then to create the corresponding termion backend:

let backend = TermionBackend::new(stdout);

The resulting code is more verbose but it works with all combinations of
additional termion features.