Support standard keybindings for scrolling terminal by one line on each platform

[Tyriar]

See xtermjs/xterm.js#215 (comment)

[Tyriar]

cmd+up/down seems to be the default scroll key on OS X, there is no custom keybinding in the preferences for it though.

[Tyriar]

ctrl+up/down is the binding on cmd. Can't seem to find one for powershell though. @daviwil would preventing ctrl+up/down from being handled by the terminal by default cause any issues for powershell?