[Console] Add autocomplete as you type

Bug fix: no
Feature addition: yes
Backwards compatibility break: no
Symfony2 tests pass: yes
Fixes the following tickets: -
License of the code: MIT

Finally got around to reviving the console autocomplete code.
Is now up to date with Symfony master. Also changed backspace behaviour to remove one character instead of two.

stty stuff is a mystery to a lot of people, so I've commented verbosely.

See also: https://github.com/symfony/symfony/pull/2364