Support xterm's `modifyOtherKeys` option for keyboard input

Full details in this PR: https://github.com/gnachman/iTerm2/pull/256#issuecomment-263117368

This should be implemented in code.

Please see https://bugzilla.gnome.org/show_bug.cgi?id=730157#c1.

Thanks, Egmont!

I'd like to support libtermkey's standard too when I eventually get around to rewriting keyboard input support. Does Gnome terminal have any plans to support it?

Nope, I don't think so.