Twin is a text windowing library.

# Usage
* Open a screen using `twin.NewScreen()` or one of its friends
* Use `screen.SetCell()` to draw characters in an off-screen buffer
* Use `screen.Show()` to send the buffer contents to the terminal
* Listen to events on the `screen.Events()` channel, for keyboard mouse and
  window resize
* Close the screen when done using `screen.Close()`

Twin opens an alternate screen buffer that it draws into.
