Xterm.js: Viewport should only scroll down on input, not when the pty writes

[Tyriar]

Similar to gnome-terminal, if the user is scrolling up to look at past output and the currently running program adds output to the terminal, the viewport should not scroll and interrupt what they are looking at.

Details

• Browser and browser version: Chrome 52
• OS version: Ubuntu 16.04
• xterm.js version: d2cef6f29db8e4e47b82000f8415b849a7756728
  

  Steps to reproduce

1. Run while [ "true" ]; do echo "ping"; sleep 1; done
2. Scroll up, notice the viewport is moved to the bottom every second

[jerch]

This is maybe another candidate for a customisation setting as it is in xterm.